“Hey Jude” is one of the Beatle’s most recognized songs. It was written by Paul McCartney, and evolved from “Hey Jules,” a song that was written to comfort John Lennon’s son, Julian, during his parent’s divorce.

It spent 9 weeks as number 1 in the United States-the longest for any Beatle’s single. At 2:58 of the song, someone can allegedly be heard saying “F*cking hell!” Sound engineers Ken Scott and Geoff Emerick claim that it came from McCartney when he screwed up his part on the piano.

It was Lennon’s idea to leave it in the final mix, buried just low enough that it can barely be heard. “Most people won’t even spot it,” Lennon allegedly said, “but we’ll know it’s there.”