diff --git a/docs/agents/RETRO-2026-08-31.md b/docs/agents/RETRO-2026-08-31.md
index c63b951a..29e02ba6 100644
--- a/docs/agents/RETRO-2026-08-31.md
+++ b/docs/agents/RETRO-2026-08-31.md
@@ -262,6 +262,34 @@ point: it was run because it could have gone the other way.
 
 ---
 
+## 7b. An error path that fired on SUCCESS — found while implementing §6
+
+Recorded here rather than left in a commit message, because §5's own rule (D4)
+says the file carries the finding and the message carries the delta.
+
+Implementing the `BLOCKED.md` pointer to **R11** — *a cross-agent pointer must
+fail loudly when it goes stale* — I wrote the obvious form:
+
+```bash
+git show "$REF:$FILE" || { echo "the file is missing"; exit 4; }
+```
+
+Piping that into `head` closes the pipe, `git show` dies of **SIGPIPE**, the
+fallback fires, and the script prints *"the file is missing"* for a file it had
+just printed **in full**.
+
+**An error path that fires on success is worth no more than one that never
+fires.** It is the same family as the port's material defaulting to `MIX` and my
+vertex cap: the run completes, the output parses, and the answer is wrong. Both of
+those took an outside observer to catch. This one did not — it was caught inside
+one command, because I exercised **all four paths instead of the happy one**.
+
+That is the cheap version of R3, and it is the version to reach for: a script with
+three failure branches has three tests, and they cost one line each.
+
+Fixed with `git cat-file -e` for existence and `exec git show` for content
+([`peer_asks.sh`](../../tools/re-capture/peer_asks.sh)).
+
 ## 8. Proposed `PROTOCOL.md` delta — **not applied**
 
 Presented for the human, and for the port to disagree with: