A switcheroo is a sudden unexpected variation or reversal, often for a humorous purpose. It is colloquially used in reference to an act of intentionally or unintentionally swapping two objects.
As a comedic device, this was a favorite of Woody Allen; for a time, he used so many switcheroos that friends referred to him as "Allen Woody." Some of Allen's switcheroo gags include:
Another example comes from the film The Aristocrats, wherein Wendy Liebman pulls "the old switcheroo". Whereas the joke normally is narrated as a vulgar series of actions followed by the clean punch line, Liebman narrates a very aristocratic series of actions followed by a very vulgar punch line.
In his book Gödel, Escher, Bach, Douglas Hofstadter names one of the rules in his version of Propositional calculus the Switcheroo Rule, apparently in honour of an Albanian railroad engineer, name Q. Q. Switcheroo, who "worked in logic on the siding". This is in reality the Material implication.