curveball
n. countable B2 Upper Intermediate curve·ball
n. a situation that is very unexpected and difficult to deal with. It comes from baseball, where a pitcher throws a ball that suddenly changes direction.
n. an unexpected and difficult-to-handle situation or event. Derived from the baseball pitch of the same name, which causes the ball to move in an unpredictable arc.
The sudden change in the weather was a real curveball.
The manager threw a curveball at the team by announcing the project cancellation just before the deadline.
While the initial proposal seemed straightforward, the legal complications that arose during the audit proved to be a significant curveball that the board was unprepared to handle.
From curve + ball.