One I wrote myself. Frustrated with some bizarre bug that I struggled to reproduce, I added two popups, to two separate conditionals that should have been exclusive (either one or the other executes, but never both). The first one said "What", and the second one "the fuck". I eventually managed to get both to show up, correctly forming the expletive, but it wasn't enough to help me solve the bug. There were more bugs though, had to get the ticket list down, so I dropped it and went after some lower-hanging fruit.

A week later, our dev team gets sent a chain email with two screenshots, and a brief comment from the lead saying "This should never happen." In the first screenshot, was my "What" popup. I didn't need to see the second one.

Turns out, when I moved on to other tickets, I completely forgot I had left that code just lying there, instead of discarding it, and had even pushed it with nary a thought. Nobody else on the dev team had seen either popup. No one in QA saw them either. And so, it made it all the way to production, where one of our users (mercifully, an in-house one), promptly caught it.

The dept head thought I was committing sabotage, given the very specific set of actions one had to take to see the effect, but thankfully everyone knew that it was too stupid to do something like that via commit into repo! At the least, my frustration at the bug proved justified.