My most satisfying bug fix?

I found a core concurrency issue in this gnarly homegrown ORM and reported it to the lead devs, who (very defensively, having written the damn thing) argued that it would never pop up in a prod environment and I was stupid for even bringing it up. Theoretically, this bug could cause pretty much every foreign key to be assigned to the wrong parent, but only if multiple instances of the application were open/running at once. They were so certain it would never happen on live that they explicitly instructed me not to fix it. After all, this bug had been active for many years on a previous project and nobody complained.

Problem was, that previous project was something that only a single user had open most of the time (think: a manager). The new project was something that would be used by multiple people at the same time (think: all the employees). Once we released this new-project-with-old-orm, it didn't take long at all for our customers to start complaining.

After that, they let me fix the bug. :)