The Bug That Only Appeared on Tuesdays

It happened every Tuesday. Not Monday. Not Wednesday. Tuesday.

I spent three weeks trying to figure it out. Logs looked normal. Tests passed. The bug only showed up in production, only on Tuesdays, only for some users.

The cause was a scheduled job that ran Tuesday mornings and cleared a cache. The cache was supposed to repopulate immediately. It did not. There was a twenty-minute window where the app was serving stale data.

Nobody had reported it because it looked like a slow load. Just a moment of things not being quite right.

What's the bug in your system that looks like a slow load and is actually a structural problem?