I believe the essence of this is already apparent in this minimal version: The professor tells you there will be a quiz at the end of class, but it will be a s

I believe the essence of this is already apparent in this minimal version: The professor tells you there will be a quiz at the end of class, but it will be a surprise. Since a quiz at the end of class wouldn't be a surprise, there can't be one at all. Then, at the end of class, the quiz is given to your surprise.