This is an easy one, just came out of my mind yesterday, while discussing calculator's architecture with a friend, even if it is not related to architecture itself.
A man walks into his office. He finds the calculator opened on his laptop with 0.33333333333 written on it. Just for fun, he wants to make 1 with just one operation (he's having such a good time). So he tries to divide by 0.33333333333. The calculator output is not 1.
What's the result he got? Why? What's the correct operation to get 1?
EDIT: Just to clarify, let's assume this is google calculator, that one you can use from google search page. Se everyone can "work" on the same calculator. It makes no difference, but this removes the "maybe this doesn't work on my '77 calculator" wild card.