Here's my "slap on the forehead" story from today...
First I've got to preface this by saying that I was having connection issues when this happened so I was a bit reluctant to get involved in a hand (sucks to have connection issues factoring into your decision making process... I know I should have just quit). Anyway I'm in early-mid position and get dealt Ad-4d, get one limper in front of me so I decide to limp too (.50 cent BB, ring game). Guy near the button raises to $2. My initial thought was to fold, but then I thought if the other limper calls then I might call to see a flop with a decent size pot (was a tight table). Well the other limper takes forever and I'm starting to think it's my connection and just before he calls I click the auto-fold button. So the first limper calls and the flops comes down, you guessed it, 3 diamonds. What made it even worse was the initial raiser started betting at it, $2 after the flop, limper called, then $5 after the turn, limper folded. I could have been looking at a very nice pot.
Of course, just to rub it in, the poker gods throw out A-A-4 on the next flop.