We have a number of players here, I’m sure there are some really bone headed deaths out there. 
I’ll start: Not realising a ring of conflict worked on the pet kitten until I got killed by him.
Play forty or so Valkyries in a row, all dying the normal mildly stupid deaths. Switch to Wizard. Start game, see a weapon in the corner. Walk over, realize it’s Mjollnir. Shout triumphantly, “This Valk shall ascend, by Thor!” Pick it up, realizing as I do so that I’m playing a Chaotic Wizard. Get blasted by Mjollnir’s power. “Do you want your possessions identified?” BTW, it turned out to be a +2 Mjollnir. What are the odds of that?
Two recent ones:
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Wading into Croesus’ army at Fort Ludios . . . forgetting that a succubus had just removed all my clothes.
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A very nice Samurai is all the way down in Gehemmon (worst part of the game, IMO, simply mind numbing) and I polymorph myself into an Umber Hulk so that I can just walk through walls. I put on a Ring of Maintain Form (or whatever it is called) so that I won’t have to keep polymorphing myself. I forget why, but eventually I want to be a Samurai again so I decide the easiest way for that to happen will be to let this friendly minotaur kill me.
Of course, I forgot to take the ring off. I didn’t revert to human form. I just died.
On a related “f*cking random number generator” topic: I currently have a Barbarian down to the castle and still have not found a bag of holding or any scrolls of enchant armor. Looks like I will have to use the wand of wishing in the Castle on stupid stuff. Hrmph!