Here's a simple but genius prank that I fell for in college:

My apartment was on the 12th floor of our dorm and on April 1st someone put an "out of order" sign on the elevator. I was already late for class and didn't even realize it was April Fool's Day -- so I ran down 12 flights of stairs and was late to class and tired. I didn't find out until late that day it was a prank.