diff --git a/test-engine/client/test.mjs b/test-engine/client/test.mjs
index da1be01..f529e38 100644
--- a/test-engine/client/test.mjs
+++ b/test-engine/client/test.mjs
@@ -228,7 +228,25 @@ function checkResponse (test, requests, idx, response, interimResponses) {
       `Received ${interimResponses.length} interim response(s), expected ${reqConfig.expected_interim_responses.length}`)
   }
 
-  return response.text().then(makeCheckResponseBody(test, reqConfig, response.status))
+  return response.text().then(
+    makeCheckResponseBody(test, reqConfig, response.status),
+    err => {
+      const details = []
+      const seen = new Set()
+      for (let cause = err; cause != null && !seen.has(cause); cause = cause.cause) {
+        seen.add(cause)
+        const code = cause.code ? `${cause.code}: ` : ''
+        details.push(`${code}${cause.message || String(cause)}`)
+      }
+      const encoding = response.headers.get('Content-Encoding')
+      const context = encoding ? ` (Content-Encoding: ${encoding})` : ''
+      const message = details.length ? details.join('; caused by ') : String(err)
+      const readError = new Error(`Response ${reqNum} body could not be read${context}: ${message}`)
+      readError.name = err?.name || 'unknown'
+      readError.cause = err
+      throw readError
+    }
+  )
 }
 
 function makeCheckResponseBody (test, reqConfig, statusCode) {
diff --git a/tests/conditional-lm.mjs b/tests/conditional-lm.mjs
index eae5c2b..6d58568 100644
--- a/tests/conditional-lm.mjs
+++ b/tests/conditional-lm.mjs
@@ -114,6 +114,32 @@ export default {
           setup_tests: ['expected_type']
         }
       ]
+    },
+    {
+      name: 'An optimal HTTP cache generates an `If-Modified-Since` request when holding a stale response with a matching weak `Last-Modified`',
+      id: 'conditional-last-modified-weak-generate',
+      kind: 'optimal',
+      depends_on: ['freshness-max-age-stale'],
+      requests: [
+        {
+          response_headers: [
+            ['Cache-Control', 'max-age=2'],
+            ['Date', 0],
+            ['Last-Modified', 0],
+          ],
+          setup: true,
+          pause_after: true
+        },
+        {
+          response_headers: [
+            ['Cache-Control', 'max-age=2'],
+            ['Date', 0],
+            ['Last-Modified', -3],
+          ],
+          expected_type: 'lm_validated',
+        }
+      ]
+
     }
   ]
 }
diff --git a/tests/vary.mjs b/tests/vary.mjs
index d3daa89..93e0a4c 100644
--- a/tests/vary.mjs
+++ b/tests/vary.mjs
@@ -473,6 +473,90 @@ export default {
           expected_type: 'cached'
         }
       ]
+    },
+    {
+      name: 'HTTP cache must not serve a mismatched stored representation after a 304 response matching on a weak `Last-Modified` timestamp alters the `Vary` header',
+      id: 'vary-altered-by-304-lm',
+      kind: 'required',
+      depends_on: ['conditional-last-modified-weak-generate'],
+      requests: [
+        {
+          request_headers: [
+            //['Accept-Encoding', 'gzip;q=0.5']
+          ],
+          response_headers: [
+            ['Cache-Control', 'max-age=2, no-transform'],
+            ['Date', 0],
+            ['Last-Modified', 0],
+            ['Vary', 'Foo'],
+          ],
+          response_body: 'abc',
+          setup: true,
+          pause_after: true
+        },
+        {
+          request_headers: [
+            ['Accept-Encoding', 'gzip'],
+          ],
+          response_headers: [
+            ['Cache-Control', 'max-age=2, no-transform'],
+            ['Date', 0],
+            ['Last-Modified', -3],
+            ['Vary', 'Accept-Encoding'],
+            ['Content-Encoding', 'gzip']
+          ],
+          setup_tests: ['expected_type'],
+          expected_type: 'lm_validated',
+          expected_response_headers: [
+            ['Vary', 'Accept-Encoding'],
+            ['Content-Encoding', 'gzip']
+          ],
+          // If `Content-Encoding: gzip` is supplied as expected, Fetch will decode it prior to the
+          // text comparison.
+          expected_response_text: 'abc'
+        }
+      ]
+    },
+    {
+      name: 'HTTP cache must not serve a mismatched stored representation after a 304 response matching on a weak `ETag` alters the `Vary` header',
+      id: 'vary-altered-by-304-etag',
+      kind: 'required',
+      depends_on: ['conditional-etag-weak-generate-weak'],
+      requests: [
+        {
+          request_headers: [
+            //['Accept-Encoding', 'gzip;q=0.5']
+          ],
+          response_headers: [
+            ['Cache-Control', 'max-age=2, no-transform'],
+            ['ETag', 'W/"v1"'],
+            ['Vary', 'Foo']
+          ],
+          response_body: 'abc',
+          setup: true,
+          pause_after: true
+        },
+        {
+          request_headers: [
+            ['Accept-Encoding', 'gzip'],
+          ],
+          response_headers: [
+            ['Cache-Control', 'max-age=2, no-transform'],
+            ['ETag', 'W/"v1"'],
+            ['Vary', 'Accept-Encoding'],
+            ['Content-Encoding', 'gzip']
+          ],
+          setup_tests: ['expected_type'],
+          expected_type: 'etag_validated',
+          expected_response_headers: [
+            ['Vary', 'Accept-Encoding'],
+            ['Content-Encoding', 'gzip']
+          ],
+          // If `Content-Encoding: gzip` is supplied as expected, Fetch will decode it prior to the
+          // text comparison.
+          expected_response_text: 'abc'
+        }
+      ]
     }
   ]
 }