Sack of Gold
One day, an engineer and a mathematician walk around and meet a very rich man who tells them that if they can get all the way over to it, they are allowed to take the sack, filled with gold, standing on the ground 20 m away. However, they are only allowed to take steps of a length of maximum half the remaining distance. The mathematician knows it isn't possible to get to the sack on these conditions, so he does nothing. Then the engineer walks all the way to the sack with normal sized steps, grabs the sack, and walks back.
"But..." says the mathematician, "... how...?"
"Measurement insecurity!" says the engineer.