Paul McCartney wrote "When I'm Sixty-Four" as a lighthearted song about a man who wonders if a woman will stay with him when he grows old. When Paul McCartney turned 64 in real life, his then-wife Heather Mills separated from him and they later divorced.
It had already taken on a somber note well before that, when his first wife, Linda Eastman, died long before they reached 64.
On a slightly more humorous note, over the course of his 64th year of life, Paul had that song played and sung to him so many times he jokingly said he regretted ever writing it.
Neither John nor George would make it to 64, either, with John being murdered at age 40 and George dying from cancer at age 58.
Hilarious in Hindsight: The original name for the album was Dr. Pepper's Lonely Hearts Club Band; while the soft drink Dr. Pepper had already existed for 81 years by the time the recording sessions started (much to Paul McCartney's amusing unawareness), John Lennon would end up becoming so attached to the drink in the years after the album's release that he would import whole shipments of it and send them to Paul.
Hype Backlash: Somewhat inevitable when an album is so often regarded as the greatest of all time. Many first-time listeners unacquainted with the Beatles tend to feel that the album's dated at best, and even among those who are familiar with the Fab Four, you can find a few people who believe that, while Sgt. Pepper is fantastic, it's not as good as, say, Revolver, The White Album, and/or Abbey Road. Most people who experience this trope with Sgt. Pepper tend to feel as if The Dark Side of the Moon, Thriller, or one of the aforementioned Beatles albums are more deserving of the "best album ever" title.
Nightmare Fuel: Try listening to "A Day in the Life" in earphones on a crowded city street, especially if it's the Mono version.
"Fixing a Hole" seems to be in the film solely so George Burns, who otherwise only narrates, can have a song. There's no attempt to connect it to anything plot-related, and it isn't diegetic either.
The scene where SPLHCB go up in a hot air balloon, and it's implied that a plane crashed into it (we see it from the plane's point of view and then an explosion), and the band is now on the plane heading towards Hollywood. What?
The final Deus ex Machina appears to have no explanation, nor is one asked or expected by the characters.
You can make a compelling argument that this entire movie is this.
Ending Fatigue: The whole business with Strawberry's death means that the final quarter of the film seriously drags.
Nightmare Fuel: Mr. Mustard's fembots are incredibly disturbing. The fact that their biggest moment is singing "She's Leaving Home" does not help.
One-Scene Wonder: Earth, Wind & Fire and Aerosmith only get one song/scene apiece, yet they're the best-regarded of the bunch. "Got to Get You Into My Life" has become one of EWF's signature pieces; some fans feel they perform it much better than The Beatles did.
So Bad, It's Good: Some sections of the film do have their fans – Steve Martin's take on "Maxwell's Silver Hammer" (lyrics not matching aside), Earth, Wind & Fire's "Got To Get You Into My Life", and the opening credits montage showing the original Sgt. Pepper and his band adapting to different musical styles from World War I to The '70s, for instance.