Saw "piston trap"
Instantly thought of a funny joke I pulled on my bro.
He had a cave that went to bedrock. He was really proud of it. I rigged it with a pressure plate that was on his stairs he had made, halfway down. What it did was, when he stepped on it, it released a piston way back in a small cave I had made bordering his cave. This cave connected with his torches, by one block. Like this:
X~X X is a block
X~X ~ is water
XVX V is his torch
The piston was far enough to where he couldn't hear it, so when the piston was released, water poured out all around his cave, flooding in and breaking the torches. We were Skype'ing as we were on my server. All I remember is him going, "Uhh. Why did I hear a pressure plate click? WTF WHERE DID MY LIGHTS GO?"
He fell off his stairs, dying of fall damage. This was unintended, but hilarious nonetheless.