There are two competing stories about the source of the naughty word:
1. Paul hits a bad chord and utters the expletive
2. John puts on his headphones and they are way too loud, so the words spill out in anger
At the 2:56 mark, a ‘whoa’ can be heard just after the chord change on the second beat – in between beats. At the 2:58 mark you can clearly hear someone say – loudly – ‘f***ing hell’. What we do know is Lennon thought it would be funny to leave it – regardless of the source – and the engineers put it so low in the mix as to be nearly inaudible. That is, until you hear it – then you can never unhear it. You’re welcome.
Although the words have been become part of music myth and legend, think about how often things like this get said over really minor things – stubbed your toe lately? – so it is more likely than not this was just a random thing that occurred without anyone noticing until the song was being mixed.
In his memoir, engineer Geoff Emerick recalls Paul hitting a bad note on the piano and uttering the utterance, but Emerick had recently been replaced as the Beatles’ chief engineer due to tensions with the band and was only involved in the mix. Twenty-one-year-old Ken Scott was the engineer on the session, but he has said he didn’t initially hear the words, and therefore couldn’t be sure where they had come from.
It never seemed likely that this was Paul hitting a bad chord. The way the words sit in the mix, they are prominent and clear – if they had been picked up by a piano or room mic they would not be as clear. It’s also more likely a musician of McCartney’s caliber would’ve reacted dead on the beat on a bad chord, and a musician with the amount of studio experience McCartney had wouldn’t have carried on with a minor mistake three beats later when the big one gets dropped.
The actual story is that John put on a pair of headphones that were far too loud. This story has been verified by people who were in the studio that day, so after all these years we have the definitive answer. It's just that simple, but when John heard his gaffe on playback he insisted they remain in the final mix.
Hey Jude was recorded on 8-track (at Trident Studios in London, not Abbey Road, as Trident was the only 8-track room in the city at the time) and tracks were precious. The tape was stopped, rewound and started again with a proper headphone balance, but the evidence of John's displeasure was not erased on the retake and so it remained.
There was a total of 40 musicians on the Hey Jude sessions – the four Beatles, even though George Harrison barely played after a spat with McCartney, and a 36-piece orchestra. Thirty-five of those musicians were paid a double fee to sing the naa naa nananana’s and clap their hands at the end. One of the musicians refused to take part saying, “I’m not going to clap my hands and sing Paul McCartney’s bloody song,” and left after his instrumental part was completed.
Now that you can never unhear the words, at least you know for sure what they are and why they're there!
