I do admit that I have my bad share of unattractive mannerisms, like nail biting. But there is this one thing that really grosses me out:
When people smell their fingertips after touching or scratching a body part.
Then their eyes scan the room to see that nobody is looking but lo behold here I am witnessing everything.
It goes something like this:

and Im like: DAMN, THAT IS GROSS. STOP IT PLEASE.