chewing on your nails
I can't stand to see people chewing on their nails or cuticles. Sometimes you see them bite a piece off then spit it out. Ugh. People, think about all the filthy surfaces you've touched, and now you're putting all those germs in your mouth. And the worst is when you're introduced to somebody and they're biting their nails, then they stick out that hand for you to shake it. No, thanks!