Stupidest things one of my PCs have done? In a campaign I've DM'ed (3.0) for a first-time player, although the campaign was in its 2nd month and the players reached level 7.
Battling through a cave with a orc army - a very stupid dungeon crawl, but the best I could produce on the fly. They kill the orc chieftain... and Eberk the Dwarf (Fighter 7) takes the chieftain's head. They go back, meeting three orcs. The cleric and wizard are low on spells, the ranger watches the rear of the group. Then Eberk charges into the orcs, yelling "I've killed your chieftain, now I'll kill you!". Next round, the four orcs fly into Barbarian rage, and full attack him (since he charged right into the middle of the orcs) - downing him to -7 hp. He died three rounds later.
Then some months later:
Eberk the Dwarven Defender (Fighter 8/Dwarven Defender 5) and the rest of the group infiltrate a Lich's dungeon. They hid behind a bottleneck in the dungeon, knowing that the Lich will blast'em, as soon as they show themselves. Cleric and Wizard want to start buffing, but Eberk says "Eh, it's a Lich. He'll use necromancy, we dwarves are pretty strong, I'll take the hit, then you can blast him!"
One round and a Disintegrate (this was 3.0) later, there was only a small pile of dust.
Then, some month later, near the conclusion of the campaign:
The group was now embroiled in a coup, delivering the "death" blow to one of Nerull's personal servant (An advanced, unique balor). The group (now level 20) defeated him... and forced Nerull indirectly to negotiate with the Gods of Light. Nerull has sworn to leave the PCs in peace, and gives them - with an ironic smirk - a gift.
They open it, finding a perfectly black sphere. Eberk (again) dons hiscloak of resistence +5 and commands the (very annoyed) cleric to buff him with everything boosting saves - everyone is puzzled, then... he grabs into the sphere to find out what's inside - suddenly everybody yells no, and the dwarf says "It's okay, there should be something important... and I've enough diamonds to let you resurrect me!"
Sphere of Annihilation.
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This dwarf was RP'd as a stout, valiant and proud defender of his nation. But seemingly his heroism equalled his foolishness... - gave me a hard time *sigh*