For me it's the "Messages have been posted since you loaded this page. View them?" banner that pops up. It's especially infuriating when I've finished typing a post and I'm just about click "post reply". I mouse my cursor over to the "post reply" icon and right before I click it that message pops up and it moves the icon away from my cursor. First I feel by the board, then I start second guessing whether I should post the reply at all.