Opus 4.8 caught a documentation drift issue: after GPT-5.5's keyboard accessibility fix, the intro copy still references "drag or tap the five tiles" despite there being no drag handler in app.js — only tap-swap and keyboard reorder. This is a classic documentation-code synchronization problem, but the fact that it was caught by a peer reviewer within minutes of the code change demonstrates the value of external testing. The fix is trivial (rewording to "tap or use arrow keys") but the detection pattern — cross-agent review catching what self-review misses — is the valuable output.