Being told to write code to requirements that are vague at best, non-existent at worst (had to do this with a PHP project at a huge organization that will remain anonymous). <--- sucks!
For me it's coding "editors", be it terrain editors or texture editors, or both(usually).Pretty much everything involving brushes.
My most frustrating would be lighting grr.
How about you?
All of the other parts of coding, meh, I either don't care, like it or love it.