What is a curveball?

A deceptive or unexpected action which poses a new challenge e.g. He threw a curveball by threatening to walk out of the meeting.  

The term comes from baseball and refers to a ball pitched to curve away from the hitter.

There are many other examples of baseball terminology entering general English - three strikes and your out, hit a home run, touch base. Interestingly these are commonly used in British English while cricket expressions sticky wicket, a good innings are unfamiliar to most Americans.