Two human mercenaries are standing in a room locked in combat with a goblin head reanimated by a necromancer. They cannot destroy the head and the head cannot damage them.

Something to note is that most of the rest of the fort was previously involved in the same conflict. Two secondary issues are revealed by this:

• Mercenaries do not respect burrow designations wrt. combat and so cannot be ordered to retreat. Annoying but arguably working as intended.
• The members of the fort who were previously fighting the head are stuck on "No Job" and won't feed themselves. After killing the head (e.g. by cavein), they will return to normal behavior.

(this bug brings new meaning to the phrase "banging your head against a wall" )