I still remember how, some time ago when I was very new still, I figured this one out.

I somehow got into a raised pot with J9, and the flop was J92.. and we went all-in, and opponent flipped AA. So I was thinking "muhaha only an ace can save him now, I got this one!".. so the turn comes something like 6, good no ace! Then the river comes 2, and while I'm sighing of relief that there was no ace.. the pot goes to my opponent! Wtf? Oh yeah.. it's now AA 22 vs JJ 99.. and highest pair wins. Damn..

Hope this helps