Shakers!
I can never stand sitting next to someone who is constantly shaking his/her leg. Its such a pain. 6 years ago, during a test held in a lecture theatre, this girl was doing just that. The entire row of chairs vibrated along with her! Today, I was attending a talk. Next to me was a ... yeah, Shaker! He was such a pain.
Seriously, I don't care if u have that habit. Just make sure that I don't feel it!