Today I got to see my supervisor get pranked at work.  Compared to the pranks that have stuck in my memory this one was harmless.
My supervisor John is a well-built man who has some of the problems dealing with people that I do, and in particular over the phone.  His "right-hand" colleague Colby, for lack of better language, works with him well, plus is bright and intelligent to boot.  They mess with each other a lot.
Around 1 this afternoon, John goes out to his van to, for reasons unbeknownst to me, have his lunch there.  In his defense it is a quiet spot and the van does have A/C.  Colby happens to be able to see her through the window, and gets into one of her occasional giggle attacks because to her it just seems strange (and it was...where I work it's accepted that when someone goes on their lunch break, you don't bother them unless the building's on fire or something else equally urgent).
The pharmacy's top man comes over -- he had a question to ask someone -- and he's good at messing with people on occasion too.  He joked about locking John in his van.
So Colby grabbed the spare set of keys that was hanging on a hook (why they were there is beyond me), and remotely locked the car doors.  Then when John tried to open the door to get out, the alarm system in the van went off.  There were roars of laughter from my side of the building when that happened, although I only found it funny for about a minute.
The alarm only went off for a couple of moments before John turned it off.  Then he unlocked his door so that he could get out.  Upon his re-entry to the building, he didn't have to guess who did it.  He already knew because he and Colby were always messing with each other.
Nothing else came of it afterwards...but I knew in his heart he wasn't pleased.