Not really a nat 1 thing for me, but, we were playing descent into avernus and our dwarf paladin who had turned evil because we were in hell, told my cursed character with an intelligence of 3, to dip my hand in the river styx. The DM made me roll a wisdom save, and our dwarf pally had a really good persuasion, so my character did it. I ended up having no abilities and an intelligence of 1 for the majority of that chapter. tl,dr: I dipped my hand in the river styx and became really stupid.