The bugs that make you think are the best/worst.

Had a ghost foreign key constraint from a dropped table. Cant drop it from a non existant table.

Turns out the dev copied a file for the new table and since you can technically name those foreign keys anything you want, there were no errors when he ran it.

Also sloppy/overworked dev teammates are the worst...

Also I'm pretty sure rule 2 of programming is "Never Copy and Paste"