Blooper: A misspoken word or phrase, especially one uttered on broadcast media. In baseball, a weakly hit ball or a high, lobbed pitch. Both usages are American in origin.
Blooper in the sense of "blunder" arose about 10 years after the word was coined in the mid-1920s. According to Richard Lederer, author of several popular books on language, the baseball term was coined almost simultaneously with to bloop: to operate a radio set "in such a way that it emitted howls and whistles, perhaps an echo of our reactions to physical or verbal howlers." Both coinages were probably onomatopoetic: imitative of the radio howl and of the sound of a poorly hit ball.
Beginning in the late 1940s, live television broadcasting greatly increased the potential for verbal blunders. Michael Erard¹, in his recently published book Um: Slips, Stumbles, and Verbal Blunders, and What They Mean, writes that a radio and TV director named Kermit Shafer was responsible for making "blooper" a household word:
Shafer was the first to transform other people's flustered speaking, slips of the tongue, and inadvertent solecisms from television and radio broadcasts into gold. In his hands, a blooper wasn't just a mistake. It was a noteworthy event, a slice of everyday media life, otherwise evanescent, that he shined up for display. There was the Vick's 44 Cough Syrup commercial that guaranteed "You'll never get any better!" Or as the stumbling newscaster said, "Also keeping an eye on the Woodstock Rock Festival was New York's governor Rockin Nelsenfeller."
Shafer claimed (erroneously) to have coined blooper; he also trademarked the word. (Shafer's trademarks appear to have expired, but there are several other live "blooper" trademarks.) He produced many records with titles such as All Time Great Bloopers, Prize Bloopers, and 100 Super Duper Bloopers. In 1974 he made a feature-length movie, Pardon My Blooper!
¹Erard's surname is a felicitous near-aptronym: a slightly bloopered version of "error."