Losing a hand in poker that you from the beginning are very probable to win. Often (or so it will seem) to a worse player, who called you bet when the odds were clearly against him/her.
Novices often confuse a bad call with a call that will probably lose, but still has the pot odds
on its side. They will then moan and moan about getting bad beaten when in fact they were only slightly unlucky. A bad beat is in fact when someone makes an incorrect move against you, and gets extremely lucky.
Good player: My pocket kings got called with Q9 when flop was K-9-A and I went all in. Turn was a 9. River came the last 9. Losing to runner-runner nines. Well, that´s poker.
Bad player: I had pocket nines and went all in for 2 dollars. This rich guy called me with AKs and caught an ace. Now that´s a bad beat! I hate this game!