Here's a prank that I saw once.
One volunteer, who worked at a candy/chocolate shop, brought in a box of chocolate-covered strawberries for everyone to share and enjoy. Another volunteer, who was in physician's assistant school, thought it would be funny to wrap the box up in a cast.
So he left it like that for a couple hours, then started cutting it away. He wound up getting powder and debris all over the chocolate-covered strawberries, and nobody felt like eating them after that. Someone said that stuff was toxic.
The guy that brought that treat in didn't pay for them, but if he had, that would have been a big waste of money. As it was, it was a big waste of time, because he hand-made them all himself.
So like someone else already said, there are certain things that should be exempt from pranks. Don't mess with food, don't interfere with someone's ability to get to their paying job on time (remember that when you're freezing keys into a big bowl of water), etc. And of course, never do something that slows down or interferes with the jobs that we as FFs or EMTs are there to do.