Players not involved in the pot telling thier cards before the hand is over. Either the plain tells, or the "oh damn!", when 555 comes on the board.

People underestimating the likelyhood of bad beats, especially all-iners. You get AA a 100 times, go all-in against bigger stacks everytime, you are certain to end up with nothing.

"This site is rigged", apart from everything else, having a online poker site and rigging it, would be killing the goose that laid the golden egg if ever there was such a situation.