… when, during the course of your job, you have to ask someone to clear their cache and have to resist the urge to just point them at FAQ 160. Or the tutorial (which I've just now noticed is out of date because it doesn't cover Firefox 2).
While I'm here:
User interfaces we have hated #76
Command-line programs which, when you get the flags wrong, just spit out a usage message and don't tell you what they thought was wrong with it.