Once in a while when playing with U4e night of the dead when i get a headshot(especially on CTF-Face...)on a bot it will spawn 2 zombies simultaneously, one telefrags the other, and somehow spawns a 3rd zombie which will cause the first tele-frager to spawn a zombie. thus creating an infinite logic loop and Piles of gore splatter and gibs etc... !
I examined my UT log after the crash and found that it had spawned ~80 zombies all at once before crashing! so there's a small little bug somewhere... probable something really obscure since it only happens once in a blue moon, and i've only seen it twice.(Both times i was playing a CTF game. Once on CTF-Face, and once on CTF-HallOfGiants)