Gregory's Server
When your stuff doesn't work because some other party X has a bug in something you depend on.
When you finally find a workaround, only to discover that party Y, on which your workaround depends, has an entirely different bug that nevertheless prevents your workaround from working, too.
Sigh. It's a marvel that anything ever works at all. #dev
@J12t likewise - happens all the time. Too frequently, I start a new project wondering *maybe I should write my own library* - opt not to, then end up doing it weeks later when the available dependencies become too much of a fudge.