When customers call in and say that their computer does not work, and they leave it at that, and think that you can help them. DETAILS people. Or worse, when people send in tickets like that, no name, no phone number, no computer number, nothing. People have to learn to actually tell what the problem is and do basic troubleshooting, since we'll make them do it anyway. We can't just guess over the phone that the printer is not plugged in causing the printing not to work, when they say "my computer wont work".
Or when I'm in the middle of troubleshooting a complex issue and they keep asking why it's happening or what is the fix. One of these days I'll slip on the phone "I DON'T KNOW, THAT'S WHY I'M TROUBLESHOOTING IT, IDIOT!".
Then there's people that say they can't log in, so I go through the trouble of getting into the VPN for that customer, opening up citrix, getting into AD, then ask their username, change their password, then after all that, they say that they're logged in, but word wont open. That's not called login in.
Dumbasses. Job security I guess.
Oh, or when people drop off computers at my house or drag me into fixing their 5 year old computer, as if that's all I want to do all day.