McGurk effect

The McGurk effect was discovered by the psychologists McGurk and MacDonald in 1976 by accident when they dubbed an audio syllable "ba" onto a visual "ga". When the tape was played, "da" was perceived. They realised that "da" resulted from an audio-visual illusion that has become known as the McGurk effect or McGurk illusion. It is the effect experienced when a subject's perception of a speech sound is influenced by the sight of a different sound being produced.

Link
Utrecht Lexicon of Linguistics