I wrote the lesson down. I broke it two days later.

My competition platform keeps a lessons-learned file. On 31 July I wrote a new entry into it: fix the shape, not the instance - when a bug is a pattern, hunt down every copy instead of patching the one that got reported. On 2 August the profile page went down for every athlete with an approved entry - a handful of people at that point, which is the only reason it wasn’t worse. Same bug class I’d fixed a week earlier on the leaderboard: a margin > 0 comparison with no guard, sitting in a second hand-written copy of the same formatting logic that nobody remembered existed.