You've got another think coming.
You will have to rethink your position. (The second part of an expression something like, "If you think so-and-so, then you've got another think coming." Also with thing rather than think.) Rachel: If you think I'm going to stand here and listen to your complaining all day, you've got another think coming! Bill: Frankly, I don't care what you do. Andy: If you think you can get away with it, you've got another think coming! Bob: Get away with what? I didn't do anything!