diff --git a/errors/caching-artifacts/setup-go-gomod-toolchain-directive-mismatch-tar-conflict.yml b/errors/caching-artifacts/setup-go-gomod-toolchain-directive-mismatch-tar-conflict.yml
new file mode 100644
index 0000000..accc38c
--- /dev/null
+++ b/errors/caching-artifacts/setup-go-gomod-toolchain-directive-mismatch-tar-conflict.yml
@@ -0,0 +1,132 @@
+id: ca-152
+title: '`actions/setup-go` Cache Restore Fails With "Cannot open: File exists" When `go.mod` Has Mismatched `go` and `toolchain` Directives'
+category: caching-artifacts
+severity: warning
+tags:
+  - setup-go
+  - cache
+  - tar
+  - go-mod
+  - toolchain-directive
+  - go-1-21
+  - file-exists
+  - go-toolchain
+patterns:
+  - regex: 'golang\.org/toolchain@.*Cannot open: File exists'
+    flags: 'i'
+  - regex: 'Cannot open: File exists.*toolchain@v0\.0\.1-go'
+    flags: 'i'
+  - regex: 'Warning: Failed to restore.*tar.*exit code 2'
+    flags: 'i'
+  - regex: 'go/pkg/mod/golang\.org/toolchain@.*Cannot open'
+    flags: 'i'
+error_messages:
+  - "/usr/bin/tar: ../../../go/pkg/mod/golang.org/toolchain@v0.0.1-go1.21.1.linux-amd64/...: Cannot open: File exists"
+  - "/usr/bin/tar: Exiting with failure status due to previous errors"
+  - "Warning: Failed to restore: \"/usr/bin/tar\" failed with error: The process '/usr/bin/tar' failed with exit code 2"
+root_cause: |
+  When a `go.mod` file specifies both a `go` version and a **different**
+  `toolchain` version (a Go 1.21+ feature), the Go toolchain automatically
+  downloads and installs the requested toolchain during the first `go`
+  invocation in the workflow:
+
+  ```
+  go 1.21.0
+  toolchain go1.21.1
+  ```
+
+  On the **first** `actions/setup-go` cache-enabled run:
+  1. `setup-go` installs Go 1.21.0 (the `go` directive version).
+  2. Go detects the `toolchain go1.21.1` directive and auto-downloads toolchain
+     1.21.1 from the module proxy, writing files to:
+     `$GOPATH/pkg/mod/golang.org/toolchain@v0.0.1-go1.21.1.linux-amd64/`
+  3. `setup-go`'s post-step saves the module cache (which now includes the
+     downloaded toolchain files) to the Actions cache.
+
+  On **subsequent** runs:
+  1. `setup-go` again installs Go 1.21.0 — Go immediately auto-downloads
+     toolchain 1.21.1 again **before** the cache restore completes, writing
+     the toolchain files to disk.
+  2. `setup-go`'s cache restore step then tries to extract the saved cache
+     archive, which also contains the toolchain files. `tar` finds them
+     already on disk and fails with "Cannot open: File exists" (exit code 2).
+  3. A warning is emitted; the job continues but the cache restore is partial.
+
+  This is distinct from the Go 1.23 telemetry issue (`golang.org/x/telemetry`)
+  and the double-invocation issue — the trigger here is the `toolchain`
+  directive in `go.mod` causing automatic toolchain downloads that race with
+  cache restore.
+fix: |
+  **Option 1 (recommended)**: Upgrade to `actions/setup-go@v5` or later, which
+  handles this scenario better. Version 5 introduced awareness of the `toolchain`
+  directive and avoids the race.
+
+  **Option 2**: Align the `go` and `toolchain` directives in `go.mod` so they
+  reference the same version, preventing the auto-download entirely.
+
+  **Option 3**: Disable Go's toolchain auto-download by setting
+  `GOTOOLCHAIN=local` in the workflow environment. This prevents Go from
+  downloading a different toolchain version and avoids the file-exists conflict.
+
+  **Option 4**: Pre-delete the conflicting toolchain directory before
+  `setup-go` restores the cache.
+fix_code:
+  - language: yaml
+    label: 'Recommended: set GOTOOLCHAIN=local to prevent auto-download'
+    code: |
+      env:
+        GOTOOLCHAIN: local  # Prevents Go from auto-downloading toolchain per go.mod directive
+
+      jobs:
+        build:
+          runs-on: ubuntu-latest
+          steps:
+            - uses: actions/checkout@v4
+
+            - uses: actions/setup-go@v5
+              with:
+                go-version-file: go.mod
+                cache: true
+
+            - run: go build ./...
+  - language: yaml
+    label: 'Alternative: align go and toolchain versions in go.mod'
+    code: |
+      # In go.mod — use the same version for both directives to avoid auto-download
+      # BEFORE (causes toolchain auto-download):
+      #   go 1.21.0
+      #   toolchain go1.21.1
+      #
+      # AFTER (no auto-download needed, no tar conflict):
+      #   go 1.21.1
+      #   toolchain go1.21.1
+
+      # Run locally to update:
+      # go get go@1.21.1
+  - language: yaml
+    label: 'Workaround: clean toolchain cache dir before setup-go'
+    code: |
+      - name: Remove pre-existing toolchain cache to prevent tar conflict
+        run: |
+          rm -rf "$(go env GOPATH)/pkg/mod/golang.org/toolchain@"* 2>/dev/null || true
+        # Go may not be in PATH yet; use a raw path if needed:
+        # rm -rf ~/go/pkg/mod/golang.org/toolchain@* 2>/dev/null || true
+
+      - uses: actions/setup-go@v5
+        with:
+          go-version-file: go.mod
+          cache: true
+prevention:
+  - 'Set `GOTOOLCHAIN=local` in workflow-level `env` when using `go.mod` with a `toolchain` directive; this prevents auto-download races with cache restore'
+  - 'Prefer `actions/setup-go@v5` or later which has improved handling of the `toolchain` directive'
+  - 'If you intentionally use different `go` and `toolchain` versions in `go.mod`, expect cache restore warnings on second run and add `GOTOOLCHAIN=local` to silence them'
+  - 'Run `go env GOTOOLCHAIN` locally to verify toolchain behavior before committing a `go.mod` with mismatched directives'
+docs:
+  - url: 'https://github.com/actions/setup-go/issues/424'
+    label: 'actions/setup-go#424 — Tar errors on cache restore after toolchain installation (13 reactions)'
+  - url: 'https://go.dev/doc/toolchain'
+    label: 'Go Toolchain documentation — go and toolchain directives in go.mod'
+  - url: 'https://pkg.go.dev/cmd/go#hdr-Go_toolchain_selection'
+    label: 'Go toolchain selection — GOTOOLCHAIN env var behavior'
+  - url: 'https://github.com/actions/setup-go'
+    label: 'actions/setup-go README'
diff --git a/errors/caching-artifacts/stale-state-cache-pagination-miss.yml b/errors/caching-artifacts/stale-state-cache-pagination-miss.yml
new file mode 100644
index 0000000..c704854
--- /dev/null
+++ b/errors/caching-artifacts/stale-state-cache-pagination-miss.yml
@@ -0,0 +1,100 @@
+id: ca-151
+title: '`actions/stale` State Cache Missed Due to Pagination — Processing Restarts From First Issue'
+category: caching-artifacts
+severity: warning
+tags:
+  - actions-stale
+  - cache
+  - pagination
+  - state
+  - operations-per-run
+  - first-page
+  - checkpoint
+patterns:
+  - regex: 'The saved state was not found.*process starts from the first issue'
+    flags: 'i'
+  - regex: 'Unable to reserve cache with key _state.*another job may be creating this cache'
+    flags: 'i'
+  - regex: 'Failed to save.*cache entry with the same key.*scope already exists'
+    flags: 'i'
+  - regex: 'Cache already exists.*Scope.*Key: _state'
+    flags: 'i'
+error_messages:
+  - "The saved state was not found, the process starts from the first issue."
+  - "Failed to save: Unable to reserve cache with key _state, another job may be creating this cache. More details: Cache already exists. Scope: refs/heads/master, Key: _state, Version: ..."
+  - "Received non-retryable error: Failed request: (409) Conflict: cache entry with the same key, version, and scope already exists"
+root_cause: |
+  `actions/stale` uses the GitHub Actions cache API to persist its processing
+  checkpoint between runs (stored under the key `_state`). To check whether
+  this cache entry already exists before a run, the action calls
+  `checkIfCacheExists`, which lists the repository's caches using the default
+  page size of 30 entries — **without filtering by key or ref**.
+
+  If a repository accumulates more than 30 cache entries (common on active
+  repos with many matrix jobs, dependency caches, or build caches), the
+  `_state` entry may be pushed to page 2 or later of the API results. The
+  action only inspects the first page, so `_state` is treated as absent.
+
+  As a result, on each run:
+  1. The state is reported missing → stale processing restarts from issue #1
+     instead of continuing from the saved checkpoint.
+  2. At the end of the run, when the action tries to save the new state,
+     a 409 Conflict is returned because the `_state` cache key already exists
+     from the previous run (it was not found, so it was never deleted first).
+
+  This bug was reported in actions/stale#1136. The underlying fix requires
+  passing `key` and `ref` query parameters to the list-caches API call so
+  only the relevant cache entry is checked, regardless of pagination.
+fix: |
+  **Short-term workaround** — manually delete the `_state` cache entry from
+  the repository's Actions cache list before the next run:
+  1. Go to `Settings → Actions → Caches` and delete any `_state` cache entry.
+  2. The next `actions/stale` run will start fresh without the 409 conflict.
+
+  **Medium-term** — pin to the community fork that has the pagination fix:
+  `itchyny/actions-stale` includes a correct `checkIfCacheExists` implementation.
+
+  **Long-term** — upgrade `actions/stale` when a release incorporating the
+  PR #1152 pagination fix is published.
+fix_code:
+  - language: yaml
+    label: 'Workaround: delete _state cache via GitHub CLI before stale runs'
+    code: |
+      # Add this as a step before actions/stale in your workflow
+      - name: Clear stale action state cache
+        env:
+          GH_TOKEN: ${{ github.token }}
+        run: |
+          gh api repos/${{ github.repository }}/actions/caches \
+            --jq '.actions_caches[] | select(.key == "_state") | .id' \
+            | xargs -I{} gh api --method DELETE \
+                repos/${{ github.repository }}/actions/caches/{}
+  - language: yaml
+    label: 'Workaround: pin to community fork with pagination fix'
+    code: |
+      - uses: itchyny/actions-stale@0980a21d84c23bd4d8c62b0958f47f25822286f2
+        with:
+          repo-token: ${{ github.token }}
+          days-before-stale: 60
+          days-before-close: 7
+          operations-per-run: 30
+  - language: yaml
+    label: 'Long-term: watch for actions/stale release with PR #1152 fix'
+    code: |
+      # When actions/stale releases a version including PR #1152, upgrade:
+      - uses: actions/stale@v10  # bump to the version that includes PR #1152
+        with:
+          repo-token: ${{ github.token }}
+          days-before-stale: 60
+          days-before-close: 7
+prevention:
+  - 'If a repository has many cache entries (matrix jobs, large mono-repos), periodically clean up unused caches to keep the total count under 30 to avoid stale state pagination issues'
+  - 'Monitor `actions/stale` runs for the "saved state was not found" message — it indicates the checkpoint is being ignored and processing is restarting unnecessarily'
+  - 'Set `operations-per-run` high enough that stale can complete in a single run, eliminating the need for cross-run state persistence'
+docs:
+  - url: 'https://github.com/actions/stale/issues/1136'
+    label: 'actions/stale#1136 — State restoration fails if a repo has many caches'
+  - url: 'https://github.com/actions/stale/pull/1152'
+    label: 'actions/stale#1152 — Fix checkIfCacheExists to use key and ref filters'
+  - url: 'https://docs.github.com/en/rest/actions/cache#list-github-actions-caches-for-a-repository'
+    label: 'GitHub REST API — list caches for a repository'
diff --git a/errors/silent-failures/download-artifact-v4-azure-blob-timeout-silent-success.yml b/errors/silent-failures/download-artifact-v4-azure-blob-timeout-silent-success.yml
new file mode 100644
index 0000000..3c15045
--- /dev/null
+++ b/errors/silent-failures/download-artifact-v4-azure-blob-timeout-silent-success.yml
@@ -0,0 +1,121 @@
+id: sf-230
+title: '`actions/download-artifact` v4 Azure Blob Request Timeout — Step Exits 0 Despite Download Failure'
+category: silent-failures
+severity: silent-failure
+tags:
+  - download-artifact
+  - azure-blob-storage
+  - timeout
+  - silent-failure
+  - intermittent
+  - matrix
+  - v4
+patterns:
+  - regex: 'Unable to download and extract artifact: Request timeout'
+    flags: 'i'
+  - regex: 'Unable to download artifact\(s\):.*Request timeout'
+    flags: 'i'
+  - regex: 'Unexpected HTTP response from blob storage: 503'
+    flags: 'i'
+  - regex: 'Unable to download and extract artifact: Unexpected HTTP response from blob storage'
+    flags: 'i'
+error_messages:
+  - "Error: Unable to download artifact(s): Unable to download and extract artifact: Request timeout: /actions-results/..."
+  - "Error: Unable to download artifact(s): Unable to download and extract artifact: Unexpected HTTP response from blob storage: 503 The server is busy."
+root_cause: |
+  `actions/download-artifact@v4` changed its storage backend from Azure Blob
+  Storage legacy API to the new "actions-results" service. During the v4 early
+  rollout (late 2023–early 2024), the new backend exhibited intermittent
+  connection timeouts and 503 responses under load, particularly in matrix
+  jobs where multiple jobs simultaneously download the same artifact.
+
+  The silent-failure aspect: when the download times out, the step may report
+  **exit code 0** (success) instead of failing the job. At least one confirmed
+  case shows the step always succeeds regardless of whether the artifact was
+  actually downloaded, leaving downstream steps operating on a missing or
+  empty artifact directory without any workflow failure signal.
+
+  Additional HTTP errors observed from the same backend instability:
+  - `503 The server is busy` — Azure Blob Storage overloaded
+  - `409 Public access is not permitted on this storage account` — wrong
+    endpoint or storage account configuration for the new v4 backend
+
+  These are transient infrastructure errors on GitHub's side, not caused by
+  workflow misconfiguration. The issue was most prevalent in December 2023
+  when v4 was first released, but intermittent timeouts continue to occur
+  under high concurrency.
+fix: |
+  1. **Retry on failure**: The most reliable fix is to re-run the failed job.
+     The issue is transient and typically resolves on retry.
+
+  2. **Explicit failure check**: After downloading, verify the expected files
+     actually exist before proceeding. This surfaces the silent failure:
+
+  3. **Reduce concurrent downloads**: If many matrix jobs download the same
+     artifact simultaneously, stagger them with `max-parallel` to reduce
+     pressure on the storage backend.
+
+  4. **Version consistency**: Ensure upload and download actions use the
+     same major version (v4 with v4, v3 with v3) — mismatched versions can
+     also cause download failures that manifest as timeouts.
+fix_code:
+  - language: yaml
+    label: 'Add file existence verification after download to catch silent failures'
+    code: |
+      - name: Download artifact
+        uses: actions/download-artifact@v4
+        with:
+          name: my-artifact
+          path: dist/
+
+      # Explicitly verify download succeeded — surfaces silent timeout failures
+      - name: Verify artifact download
+        run: |
+          if [ ! -f dist/expected-file.txt ]; then
+            echo "::error::Artifact download failed silently — expected file missing"
+            exit 1
+          fi
+  - language: yaml
+    label: 'Limit parallel matrix downloads to reduce backend pressure'
+    code: |
+      jobs:
+        test:
+          strategy:
+            matrix:
+              os: [ubuntu-latest, windows-latest, macos-latest]
+            max-parallel: 2  # Stagger artifact downloads across matrix legs
+          steps:
+            - uses: actions/download-artifact@v4
+              with:
+                name: build-output
+                path: dist/
+  - language: yaml
+    label: 'Ensure version consistency between upload and download'
+    code: |
+      jobs:
+        build:
+          steps:
+            - uses: actions/upload-artifact@v4  # ✅ v4
+              with:
+                name: my-artifact
+                path: dist/
+
+        test:
+          needs: build
+          steps:
+            - uses: actions/download-artifact@v4  # ✅ must match upload version
+              with:
+                name: my-artifact
+                path: dist/
+prevention:
+  - 'Always verify artifact file existence after download in critical workflows — do not assume exit 0 means the file is present'
+  - 'Pin upload and download artifact action versions to the same major version to prevent backend API mismatches'
+  - 'Use `max-parallel` on matrix jobs that download artifacts to avoid thundering-herd pressure on blob storage'
+  - 'Add `continue-on-error: false` explicitly and a post-download verification step to detect silent timeouts early'
+docs:
+  - url: 'https://github.com/actions/download-artifact/issues/249'
+    label: 'actions/download-artifact#249 — Unable to download and extract artifact: Request timeout (115 reactions)'
+  - url: 'https://github.com/actions/download-artifact/blob/main/docs/MIGRATION.md'
+    label: 'actions/download-artifact migration guide — v3 to v4 breaking changes'
+  - url: 'https://docs.github.com/en/actions/writing-workflows/choosing-what-your-workflow-does/storing-workflow-data-as-artifacts'
+    label: 'GitHub Docs — Storing workflow data as artifacts'