Hmm, there are two kinds of stupid in my book. silly stupid and obnoxious stupid.
Silly Stupid:
This one happened in a game years ago. We were low level PCs (2nd or 3rd, cant remember which) and were heading to the docks for some sort of meeting. As we are proceeding down the street that follows the bay towards the meeting place, we notice two guys fishing. OK, nothing to suspicious about that, other than it is kinda late in the day to be fishing, seeing how the sun has set and all. We continue on and are then informed that the 2 "fishermen" are rising and drawing longswords. I think the conversation went something like this
PLayer 1: Longswords? You didn't mention they had longswords.
GM: Yeah, well, they were in their scabbard and all, you just didn't notice them
Player 1: (still incredulous but willing to let it fly) Ok, well in that case I'll throw my dagger (expecting trouble, all the PCs had readied weapons like daggers and darts concealed on them and ready to go, the GM was willing to give us an action while they clambored to their feet) Rolled a 17, which does ...
GM: Sorry, that doesn't hit.
Player 1: WHAT? how does that not hit?
GM: They are wearing full plate.
(At this point all three players looked at eachother in amazement)
Player 2: And we just happened to miss the fact that they ARE WEARING FULL PLATE!?
GM: .....well they are
Player 3: Fine, I rush them and throw myself at them in an attempt to knock them into the water.
GM: OK, they get hit and all three of you go into the water. You can swim right?
Plaer 3: Yeah, but they can't.
GM: Yes they can, both are proficient swimmers.
Player 2: In full plate?
GM:........... You continue down the docks, slightly wet after your immersion.
Obnoxious Stupid:
Played a GURPs game once that was set in the near future. My PC suddenly started being pursued by a large corporation intent on controlling him (for reasons he couldn't understand since he was essentially a small time hoodlum) and started having increasingly large periods of blank time where he would do things and not remember them (like hacking into government files and banks). Turned out I had an AI implanted into my brain, which was opperating independantly of me (hence the blank times). It ultimately helped the GM railroad my PC into doing things I didn't want to do. Worst part about it is that the whole setup should have been included in the characters flaws, thus allowing me to purchase benifits at PC generation to offset them. Only redeeming part about it is that the GM now agrees the game was horrible and learned a lot from the game.