When your part of the code stops working because someone else changed their part of the code. Often their functions take more arguments than they did before. Sometimes they are eliminated entirely or placed in a different file.

I also dislike merges because it's like, you want to change the code this way, I want to change the code that way, so how should we change the code? I've always been able to find a way to incorporate both our changes, but if there was a real conflict it would be an awkward process.