I'm not sure this is quite up to horror stories, but its legit the first session I can remember not being fun with this DM. Last session we'd gotten level 19, so I was looking forward to having some interesting challenges. And this campaign generally has been pretty good, but this session was just so different than the rest of the campaign. Admittedly most of the campaign was the Vecna module and this is something the DM homebrewed to change some of the end storyline.
Session started with a trap with a homebrew giant ooze that took 1 damage from anything, could ooze past a wall of force and any any other magic, but the magic sword he'd given his son 2 sessions ago killed instantly using its special ability. This was arguably the only attack roll that mattered the entire session.
Next thing was a 'trap' on a door, where when someone read the plaque on the door we were all handed an AI generated badly written story of a bunch of adventurers going in to a tomb. We were then put in control of a set of kobolds, ogres, and skeletons, told we could place two traps, and challenged with killing the party of 5th level adventurers off. This devolved in to a roughly 3 hour combat that took most of the session. And part of the problem is the DM took so long on the "adventurer's" turn because he'd written them all up like PCs.
Then we found 4 jail cells. Two of them had a polymorphed copy of the same one of one of our main allies -- that true seeing could not see through. And the npc's couldn't explain how they got polymorphed in to looking like our ally, or what they were trying to accomplish. I think the DM thought them being that way would be some sort of puzzle? Like with all the other AI in this session, I'm wondering if this was just an AI thing.
And then we didn't get to the final fight he'd planned so he just gave us the AI generated intro text, which was a rip-off of dungeon crawler carl.
Literally for a first session post level up, I spent most of my session playing a pair of kobolds and dealing with AI generated crap.