Received the text:

An internal error has occurred:
Stacking monsters (an invisible stalker 74/ {1073741824, 0} [-1, -1] with a stone snake/171 at (72, 8) is not allowed (L: 9, 2; LI: -1).
I'm trying to fix this by simply deleting the invisible stalker.
If it's in important monster, it might still ruin your game.

This then repeats over and over again. The monster name in the first block (stalker) changes each time it loops through. Second one is always stone snake. All else stays the same. Had to click ADOM closed as the loop went on ad infinitum.