What's a bug that took way too long to figure out because users never reported it?

A few years ago I would've said crashes are the worst bugs. Now I think it's the opposite. The worst bugs are the ones where everything technically works. The request succeeds. The button works. The API returns 200. Nothing shows up in logs. Meanwhile users are getting confused, clicking the same thing repeatedly, or abandoning a flow entirely. By the time someone notices, it's been happening for weeks. What's the most frustrating example of this you've run into? Edit:- I am not getting the point why everyone is downvoting this post ? It's a serious question guys submitted by /u/Icy-Roll-4044 [link] [留言]