OK before I vote I gotta explain it. Yes I've done it. I was in high school, and three friends (two gals, one guy) dared me to sneak up on someone else (also a close friend, mind you) and kiss him. I declined at first, but I changed my mind after those chicks started that female "You will do it" attitude.
Anyways, we entered the class room, and my target was looking down into his binder. His girlfriend (which is in on this) snuck up on him and said, "Hi Mike". Of course he expected her, but instead I came between them and quickly kissed him.
The look of utter shock and complete surprise on his face.
Priceless. That alone made it worth it. rofl It was a classic. He just sat there for the next 10 seconds, completely frozen. Then people started laughing. Not just us 4, but everyone in the classroom. Now if you are thinking this, yes of course I was embarassed. It was quite possibly one of the most disgusting things I've done, but the girls in the class thought it was hilarious. He came to his sense shortly and pointed an accusing finger at his girlfriend, with that oh so subtle voice, "You put him up to this, didn't you!"
He vowed revenge.
One of the better practical jokes we pulled.