The Beatles may have their roots in Merseyside but the sounds emanating from Abbey Road Studios during the mid-1960s might as well have been beamed down from another planet, unlike anything else that had been heard before.

Nevertheless, being a trailblazer certainly comes with its challenges. Namely, there is nobody to compare yourself to. For much of their existence, The Beatles were out on their own, cooped up in a studio staring at the same old faces. On one hand, this relative isolation - save for the occasional inclusion of session artists like Nicky Hopkins, Billy Preston, or Eric Clapton - spurred on the creation of some of the most revolutionary albums of the 20th century. On the other hand, though, there was nobody to ease the band's uncertainty about some of their more experimental, out-there offerings.

It's worth remembering, after all, that the age of Beatlemania was one of loved-up teeny-bopper pop songs. The likes of 'Love Me Do' were revolutionary, sure, but they weren't a million miles away from the pop charts of the early 1960s. It was a far cry from the drugged-up, intergalactic psychedelia of records like Revolver, and no band before - or, arguably, since - has managed to make that pretty colossal switch-up in sound look as seamless as The Beatles.

With that leap into the psychedelic unknown, The Beatles were blazing a trail that countless others would follow, but that didn't mean that the band never doubted themselves. Paul McCartney, with his penchant for Edwardian music hall ditties, initially lacked a little confidence in the far-out sounds of albums like Revolver, for instance. Despite being up there in the rankings of The Beatles' greatest albums, the album that firmly established the band's expansive new age was the cause of a lot of anxiety for Macca back in 1966.

As the songwriter recalled during an interview with 60 Minutes back in 2018, "I got the horrors one day. I thought it was outta tune. I thought the whole album was outta tune. I listened to it and for some reason just, like, 'Oh my God.'" Fortunately for all, though, McCartney's worries were quickly quashed.

"I went to the guys, I said, 'It's outta tune. It's outta -- I don't know what we're gonna do,'" the bassist remembered. "You know? And they said -- and they got a bit worried and listened to it. They said, 'No, it isn't.' I go, 'Oh, OK.'" Problem solved. Admittedly, the idea that all four members of the band, plus producer George Martin, might have missed the album being out of tune is pretty ludicrous, but McCartney's "horrors" perhaps reflect some of the anxieties surrounding the album.

Nobody had heard anything remotely close to Revolver before, what with its newfangled production technologies, Eastern influences, and palpable psychedelic overtones, and it marked a stark departure from the comparatively mellow 'Can't Buy Me Love' or 'Eight Days A Week' that had come before. So there was no telling how audiences would react upon hearing it, and that could easily have caused some concern for McCartney during its production.

Luckily, the album was absolutely in tune, and quickly became one of the defining albums of the 1960s, so McCartney needn't have worried.