Retroscript

Retroscripting is the practice of recording new or unscripted dialogue over a live-action or animated program. It may be done for a number of reasons:

• To capitalize on a late-breaking news story or scandal and make the episode current,
• To censor potentially troublesome or possibly insensitive dialogue. An episode of Desperate Housewives suffered this fate following the passing of the Pope. A line was redubbed from "Boy, those Catholics can really mourn" to "Boy, they can really mourn", and it's painfully obvious from watching the actress' lip movements. The potentially offending line was uttered at a funeral procession. "They" referred presumably to the Solis family, who had just lost Carlos Solis' mother to tragedy.
• If an actor has repeated trouble with a line or word. In one episode of Star Trek, according to William Shatner, one of his fellow actors had no end of trouble pronouncing an alien creature's name. It was "Gumatu", but the poor fellow kept saying "Mugatu". The monster, a huge Sasquatch-esque savage, eventually became officially known as the Mugatu.