Fighting the Grigori. They have an Adventurer promoted into an Immortal in their capital. Every time I killed him in the capital he healed back up to 1/4 life or so. Infinite times per turn. (Well ok, I only tried five or six times in a row on a couple turns in a row, but the behavior didn't show any sign of slowing down.) Had to go into the worldbuilder to remove him.
I have a save game where this is reproducible if needed.
I got it reproduced here, thanks. It will be fixed in 0.33.