Allen Holub just published on SDTime an editorial named "The Terror of Code in the Wrong Hands". I like the description of the "software terrorist": the guy who stays up all night, unwittingly but systematically destroying the entire team’s last month’s work while “improving” the code. He doesn’t tell anybody what he’s done, and he never tests. He’s created a ticking time bomb that won’t be discovered for six months.
I am sure we all have a small list of co-worker that we put in this category... Hoping that I am not in yours :-)