Not coding, but similar effect....
The admin console of Novell Networks allowed for the sending of IM's to people. Didn't get used very often, except for a couple of the PFYs to annoy each other. One day this was occurring, so one PFY logged off to (he thought) put a stop to it.
Unfortunately the IMs are based on user IDs assigned at login, and when someone logs off that ID is often immediately assigned to the next user that logs in, if it's within a few seconds.
So ……..that was why a senior manager called up to ask why his PC has a message in the middle of the screen saying "STOP IT!!!!".
Could have been a lot worse though!