Seeing that I worked as a lab consultant at my university, some of this stuff would never happen.
Most of our equipement is pretty well bolted-down with each station having a low partition between the computers on either side.
Screaming out things like "You will all perish in flames!!!" will result in a student's immediate ejection from the lab and perhaps even a nice visit from the campus police.
With that said, the other employees and I would sometimes have fun with our friends who were using the lab by sending bizarre messages to their computer which would pop up onto the screen as a dialog box. Things like, "Quit staring out the window" or "Are you wearing clean socks?"
.....Kelly