Updates! Updates! Updates!
Firefox updates that break it, or break a particular addon you rely on (I've switched them off). Constant itunes updates, security updates that break your entire system, or stop some part of your software from working (lucky I don't use Windows much), toolbar updates, addon updates, full program updates that you have to do or they stop it being compatible with something else that's just been updated...
Aaarrrgghhhh... :-)