Buddy of mine used to work IT for some group or another back in the day. Had edited himself a version of DOS (like I said, back in the day) so the error messages were a bit...altered. You know, like "Bad user. Stop doing stupid things." and "No! You're stupid!". For some reason, it wouldn't compile right, so he tossed it on a floppy disk and forgot about it.
...one of his co-workers found the disk, thought it was a regular version, and installed it on the head honcho's computer. An hour or so later, he gets a call asking to come up and inspect the computer to find out why it's calling the boss an idiot.
Unintentional, but one of the best.