A few examples would be:
- Badmouthing or insulting someone while they're within earshot. It would be nice not to insult anyway, but if one HAS to say that "Jenny's new skirt is sooooo last century!" try not to say it while poor Jenny is standing a half a foot away. xP
- Interrupting others. It's just annoying. And on the other hand, don't talk forever without giving others a chance to respond.
- Mixing whites and colors in your laundry. (<_<')
- If you see someone inching their way away from you as you're talking, they're ready to end the conversation. In similar idea, if you notice someone is becoming suspiciously quiet, or is only making grunting nioses when you ask a question, they're finished. Some people don't realize that.

[/rant]
Share some of your own here.