I jump at loud noises. 

Not a little jump or anything, but a ridiculous leap of terror. If I'm in a quiet room and a loud noise sounds, you'd think I was taking shelter from gunfire. I was at a party once, and someone decided to pop a bunch of balloons. I did my jumping thing at every single pop (at least ten in total), even though there was already loud music playing. I ended up having to leave the room because I got sick of instinctively jumping around to face the source of the popping every damn time it happened. XD