I got a couple for you...
Saw this one a few times back in the dorms. It works very well on a tile floor. Piss in a ziplock bag, lay it flat and freeze it. At some point, usually during the night, slide it in under the victim's door. It will slide into the middle of the room and when said victim awakes in the morning he will discover a smelling puddle of piss in the middle of his floor, usually by stepping in it, and he will have no idea how it got there.
This one is a story of a prank gone bad.
**WARNING WARNING WARNING** Don't try this at home. In fact, I probably shouldn't even tell of this attempt of mine to win a Darwin Award. But what the hell, in the sake of "educating others" on what not to do....
The situation:
So once upon a time, many semesters ago, I was living upstairs in the fraternity house. At the top of the stairwell was the required EXIT sign that we are all familiar with. Well, during one drunken party, someone reached up and hit the sign, knocking it across the room and exposing the two hot 120 volt A/C electrical wires. Well they just hung there for awhile until sometime in the future when "someone" got the bright idea and found the balls to touch them with two of his fingers on the same hand. You'd get a pretty good shock, but it wouldn't go past your mid-forarm and seemed like a pretty harmless little stunt and many others tried it as well. Well it didn't take long until that progressed to touching the wires with different hands and getting the shock through the core of your body. That is a unique feeling. Many others tried this as well with no problems, even progressing to making chains of ten people or so and the two people on each end touch a wire and everyone getting the shock. Good times, I think it was frying our brains....
Now the prank:
"Someone" then went to the hardware store and bought two long strands of copper wire. Those two wires were affixed to the backside of the handrails going up and down the stairwell, one on each handrail. They were then attached individually to each the positive and negative wires hanging out of the wall. That way when some poor unsuspecting soul would grab the handrails, they would complete the circut and get quite a surprise. "Someone" tested this out themselves first of course and it worked splendedly. Several people were shocked very good and their yells of surprise delighted us greatly. But all these prior examples of us shocking ourselves had one thing in common, we were all standing with both feet on the floor and as soon as we would touch the wires we would jump and let go. No one locked on. And then some unlucky dude was on his way downstairs in a hurry and in one motion, leaned forward and grabbed the handrails while swinging his feet down 5 or 6 stairs. Because he was midair in the swing down when he got the shock, he had gotten a really tight grip on the handrails, he couldn't let go fast enough. And then instantly, he locked on and couldn't let go. Well after he nearly ripped both arms out of his sockets because he couldn't let go, he started screaming while stereotypically shaking violently. Yeah well... the pucker factor kicked in for those of us watching and no one moved for a second. Then very quickly "someone" ran down the stairs and knocked him free. Total time this dude was locked on was probably about 2 seconds. The fella was fine, but very very pissed off. Had he been a bigger dude, we all would have gotten our deserving ass whopping. Instead he cursed us all out while leaving the area very quickly. "Someone" bought the guy a lot of beer the next day and we are still friends to this day. Sadistic, Mornonic thing to do? Yes. Funny as all hell and still the best prank I've every seen? Yes.