Oh, I've had players in my group do stupid things before. I loved the dwarf who walked into a disintegration field without doing a proper search of the area before. (They'd detected it as a strong transmutation aura, and it was giving off bad vibes). He failed his save, and poof, they were a PC down. If they'd done any searching of the area first, they'd have found the secret way past the field.
Then there was the illusionist who, in the same adventure, cast a
fireball spell on a bunch of commoners when several powerful undead were sitting on a stage in plain sight. The kicker was that he didn't tell the other players that he was about do do that - they'd been infiltrating the crowd and were getting nearer to the stage. Suddenly their element of surprise was lost and that was a TPK to me!
There was the 10th level barbarian who charged a group of 8 hill giants... without checking to seeing what the rest of the group were doing. (They were casting invisibility and haste on each other). By the time they entered the battle, a couple of rounds later, the barbarian was dead.
Mind you, they do play intelligently and excellently well from time to time as well...
There was the Priest of Trithereon (god of independence and freedom) who had to sit through a banquet with slavers, with a Priest of Hextor telling him that slavery was the will of the gods. He had to remain undercover, and only just managed to keep his temper under control.
There was the player who managed to remember the way out of the dungeon, after their mapper had been
teleported elsewhere (along with the map), even though I'd used revolving rooms, false stairs and elevator rooms to confuse them all... he just took the perfect way out.
Of course, my favourite dumb thing of all time: the rogue who decided that one of the guests at the slaver's dinner was actually a slave. (She wasn't) He then "rescued" her. (She played along). They became lovers, and eventually he led her to the secret stronghold of her bitterest foe. "Thanks, my dear. It's been fun!" she said, before teleporting away to tell the rest of the Scarlet Brotherhood where to attack...
Cheers!