In a 5e converted Tomb of Horrors...
Setting the scene: This is the tomb of a lich. There’s a hallway filled with orange mist in front of us. The Paladin went down the last unknown hallway so he brazenly decides to go down this hallway first too, while the party waits in the room to see if he gets insta-killed.
Dm: You go down the hallway until you reach a room. Where, your Chaotic Good alignment becomes Lawful Evil, and you become biologically female.
-lots of OOC jokes about how we are no longer a sausage party-
Paladin (OOC): Well lawful evil me doesn’t want to risk any of these people becoming violent chaotic dickheads so, I walk back like everything is fine.
Dm: As you pass back through the mist, your alignment returns to Chaotic Good, but you stay biologically female. Nothing short of a Wish spell or Alter Reality can change it. Blame Gary Gygax.
The party, in character, when the paladin returns: Dude, what happened? What was in there? Are you okay? etc
Paladin: *deep sigh* The great evil of the lich that pervades this dungeon, it just, *clenches fist*, grabs you by the balls.
-Everyone goes nuts-