Curve ball.
This idiom or expression refers to:
A tricky situation.
A deceptive action.
A. How can you blame her? She was thrown a curve ball!
B. I know but, in her position as the chairwoman, she should have been more prepared.
History:
The term comes from baseball. A curve ball is thrown in such a way that it follows a curved path in the air. This, combined with the high speed at which the ball is going, makes it difficult or tricky for the opponent player to handle.