Let me ask you a couple of questions:

1) Earlier in this thread someone mentioned a prank gone wrong where a coworker failed to have his laptop available for a PowerPoint Presentation and had to work without it for a client because it was "stolen". If that client then took his business - a multi-million dollar account - elsewhere because of their perceived poor preparation of your company, would you tell that client to "lighten up"?

2) If you used the fake BSOD screensaver on a coworker and he/she got frustrated and did real damage to the computer thinking it was already dead, would you tell the company owner, who must now shell out the money to fix that computer, to "lighten up"?

Pranks aren't meant for the workplace. They can, and do, go disastrously wrong. They waste company resources, inhibit productivity, and can cause a very uncomfortable work environment for those that don't get "kicks" out of them. Overall, they are in poor taste and reflect a very low level of professionalism.