What broke in your product without telling you, and how long until you found out?

had one this week. a framework update changed how apps launch, and one function kept working exactly as before except the data it hands you is now empty. no crash, no warning, nothing deprecated. deep links just silently stopped opening.

took me a while to find because nothing failed. it all looked like success.

made me realise almost every expensive bug i've had was this shape. quiet. the loud ones get fixed in ten minutes.

so what is the one that got you? and what made you notice in the end, a user, a metric, or luck?