I didn't play muds and my experience was mostly limited to helping him fix C programming bugs from time to time and fielding an occasional irate phone call from users who got my number off the whois data. But because of the programming help I had some kind of god-access to the mud.
One afternoon I had a ladyfriend over that I was presumably trying to impress and she'd asked about the mud. We hopped on and I summoned into existence a harmless raggedy-ann doll. That was kind of boring so I thought it would be fun to attach an NPC script to it, -- I went through the list and saw something called a "Zombie Lord" which sounded promising. I applied it, and suddenly the doll started nattering on about the apocalypse and ran off. Turned out that it killed everyone it encountered, turned them all into zombie lords, creating an exponential wave of destruction that rapidly took over the over the whole game.
I found the mental image of some little doll running around bringing on the apocalypse to be just too funny-- until my phone started ringing. Ultimately the game state had to be reported to a backup from a day or two prior.
[I've posted a couple examples, -- I dunno which one is best, but people can vote. :)]