Last year, a beautiful opportunity presented itself the day of. One of my co-workers wanted to play a joke on his labmate. The labmate had had her wallet stolen and her ATM card used, but security wasn't able to identify the perp from the ATM's camera. So, I sent a fake e-mail from security, including the name and number of the investigator, Josh Inyoo, stating that using some new technology they were able to get better resolution on the photo and wanted the victim to see if she could identify the perp (using the fake doctored photo of my friend). She completely fell for the stunt, and her lab (and security, who got a couple of phone calls from her asking for Josh Inyoo) had a good laugh at her expense. (She laughed too when the stunt was revealed.) You can read about it here.
Alas, I wasn't able to pull off or attempt any other practical jokes -- none at all, sadly -- despite my desire to do so. And this year, it looks like I will go jokeless...unless of course I become the victim of a stunt myself.