If you have a planned multi-turns move for a stack, and this stack is attacked by monsters or hostile AI, the stack will complete its move for the turn.

The problem is, it could finish this move on an already occupied tile without combat... thus clearing a  lair without defeating its guardians.

Example:

I spotted a death demon lair in the central desert of 'Dust Bowl'. I order my stack to attack the death demon itself; the stack completes its move for this turn and needs one more turn to proceed with the attack.

Press End Turn.

I got attacked by wildings . Once dispatched, i see my stack moving from its own initiative, ending on the Death Demon Lair STILL OCCUPIED. I got the 'lair cleared' message and item.

Tadââ ! My stack is now standing on the same tile than an apprently not-so-deadly Death Demon. Which did NOT attack...

 I think the problem is due to how the game handles AI moves in parallel.

I have lost the saved game for this particular situation, but it seems to happen quite frequently when any of your stacks is Under attack.