I was playing on hardcore mode and I encountered several skeletons in a cave deep underground. I was able to cause all of them to fall into lava in their turn. All but one died. The surviving skeleton just stayed in the lava, one block deep and on fire, and showed no signs of taking damage and didn't die. It continued shooting arrows at me as normal. When I returned it appeared as though I hadn't dug an entrance into the cave at all.
What I expected to happen was for the skeleton to burn up in the lava and die and for the cave entrance to remain.
What actually happened was that the skeleton didn't take damage or die and otherwise acted normally, and that the cave entrance vanished.
Steps to Reproduce:
1. Play on hardcore.
2. Find a cave with lava and skeletons like mine.
3. Have one fall into lava and survive.
4. Restart the game a few times.
5. Leave the cave and return later.
Cannot reproduce, please force a crash by pressing F3 + C for 10 seconds while ingame and attach the crash report here.