curve ball

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Baseball Any of several pitches that veer to the left when thrown with the right hand and to the right when thrown with the left hand.
• noun Slang Something that is unexpected or designed to trick or deceive.
• idiom (pitch/throw) To mislead; deceive.
• idiom (pitch/throw) To cause to be surprised, especially unpleasantly so.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a pitch of a baseball that is thrown with spin so that its path curves as it approaches the batter