The most annoying thing I know is when people do not reply to my mails. I know it's usually a trivial matter to them, or they're busy or deem the mail not good or in need of an answer.
It's a personal thing, but whenever I mail someone something, and they won't reply, I get the feeling that the person is either mad at me or I did / said something wrong. It's a horrible feeling, and not a very rational one. I don't know what the point of that is, but I do have it.
And this is not meant for TET or anyone here, just needed to vent and this thread has a similar topic. Sorry Gameon for derailing this thread a bit...
The best ones are when someone mails you for support, you either ask for more details to be able to help or give them a possible solution, and you never hear from the person again. I've grown to ignore these people, but it sucks because they leave me with unresolved stuff and that's not good in front of the boss.