Pranks are a sign of trust. The trust bit is that you only use it for pranks and not any of the myriad other things you can actually do with a logged in computer. But the pranks should be a reminder they might just as well not have been a prank, and that you should bloody well lock your computer already.
Can't find it at the moment but I remember Adam Savage once saying that him and Jamie Hyneman never play practical jokes on each other. It would end up with one of them duct-taping the windows on the other one's house to fill it with water. Don't prank your sysadmin. It won't end well for you.
For context, the two Mythbusters presenters have famously different personalities and are not exactly friends. This reinforces the idea that pranks are generally done between people who feel they can trust one another.
There was trust between them. Trust does not require compatible personalities. Both of them have spoken at length about their working relationship. Part of that trust was knowing the other one would never condone or sign off on pranking. Adam was pranked once with a cattleprod and it resulted in firing the producer that was responsible (and Jamie was not in on it).
Pranks are cheap laughs, slapstick humor. It does not belong in a work environment.
