The term for a figure of speech in which a sentence or phrase has an unexpected or surprising ending. Often used for humourous effect, and thus heavily used by comedians.
“Where there’s a will, I want to be in it.”
"If I am reading this graph correctly – I would be very surprised." — Stephen Colbert
"If you are going through hell, keep going." — Winston Churchill
"I sleep 8 hours a day. And at least 10 at night." — Bill Hicks
'The car stopped on a dime, which unfortunately was in a pedestrian’s pocket,' said one.
'A witty paraprosdokian there, my good fellow,' said the other.