Play online poker for any length of time and you're likely to face the menace of the All In Man(who of course can also be a woman). The All In Man does what his name implies, he goes all in, that is bets all his chips in games where it is allowed, hand after hand, usually before you see the flop. And unfortunately All In Man tends to get paid off for it. Players who don't get premium hands will fold to him, allowing him to steal the blinds and whatever anyone has bet in front of him. Those that don't fold those less than premium hands frequently end up on the short end of the stick against him. If he doesn't get knocked out one or two hands into the game he'll soon have a good enough stack that there's nothing to discourage him, as even if he looses a hand he'll have enough chips to survive all in moves against smaller stacks.
What's even worse is when you get a good hand and he still beats you. I had this happen to me tonight. I was playing in a freeroll on Pokerstars and had built up an alright stack. I ended up on a table with All In Man and folded several hands as a result. Then I got QQ and decided I'd call his all in. After all someone who all ins every hand can't have great cards every time. With only two of us in the hand his hole cards were revealed as 4-5 suited. But as too often seems to be the case with these guys he ended up catching an A to 5 straight and it was byebye for me. Frankly I really should have known better. A better strategy would have been to fold that hand and wait for either someone else to take him out or for the server running the tournament to move one or the other of us to a different table. It was early enough in the tournament, and I had a big enough stack, that I could sit out the blinds for a while. But those ladies were just too tempting.