I think this is one one my favorites, I came upon it when I was taping my friends chair. (It had a huge gash on the side NOT FUN)
Step 1: What You Need
1. Lots of clear tape
2. Scissors (to cut the tape)
3. Unsuspecting person
Tip: You should do this about an hour before the person comes just to make sure tape is strong enough to hold.
Step 2: Getting Ready
The lever that adjusts the chairs height you need to tape it in the up position so when sat on it will sink down very quickly, kinda like quick sand.