"The rating is student preference, not capability."  A rule about the words in the interface, checked by a person before release — the leaderboard may never be called a benchmark.

"Both bad" does not update the rating.  It is a failure signal, not a relative one. Put it into the rating and it quietly breaks the scale.

Mechanism: the "already processed" marker was written after the work. A failed concurrent write meant the batch came back — a loop feeding itself. A table written months earlier to fix precisely this had never been applied to production.
Lesson: claim work before doing it — and a migration that never reached production does not exist.

Mechanism: a constant rendered into the page at load time, in an interface that then switches workspace without reloading. Every later request carried the workspace the user had already left. Not a breach — both belonged to the same user — but the work, the logs and the cost went to the wrong one.
Lesson: render-time constants are a time bomb in any stateful client. Side effects must sit after every gate. Otherwise a rejected request still leaves debris.

Mechanism: a dependency vendored by its "latest" tag resolved two major versions below what the code assumed. The API had changed shape, so every call quietly did nothing — for two months.
Lesson: pin versions, and treat silence as the expensive failure — a crash gets reported, nothing happening does not.