The Beatles' songs are filled with hidden quirks and self-references including a convoluted series of clues that claimed that Paul McCartney was, in fact, dead  – the most famous example being the cover of 'Abbey Road' where he is walking barefoot, out of step and holding a cigarette. But the hit single 'Hey Jude' has an altogether different secret that could've killed its chart success if anyone realised at the time.

At about 2:58 of the song, McCartney can be heard to say, "Oh, ******* hell!", but Lennon liked it so much wanted to leave it in. Sound engineers Ken Scott and Geoff Emerick said: "'Paul hit a clunker on the piano and said a naughty word,' Lennon gleefully crowed, 'but I insisted we leave it in, buried just low enough so that it can barely be heard. Most people won't ever spot itÂ…but we'll know it's there.'"