I once wrote an (Amylaar LPC) enemy who, if you attacked him and he was near death, would summon two comrades to fight for him.

Unfortunately, they're immediately hostile so you start attacking them too, and _they_ get near death and summon two comrades each, and so on. It turns into a Sorcerer's Apprentice scenario very quickly.

I managed to bring the MUD to a halt. An archwizard (not Yen Sid) had to instakill the enemies to avoid hitting their "near death" condition and creating more of them.