During the record company convention scene, the band members have herpes sores on their lips. The sores weren't there before. In a deleted scene, the band tours with an opening act, each band member has sex with the opening act's lead singer, and the entire band gets herpes.
During Big Bottom, just as David sings "The looser the waistband", his girlfriend Jeanine appears on the left edge of the stage. She joined the band much later in the film, so she shouldn't be there (Widescreen only).
When the band goes to the recording industry convention, the subtitles say they're in Atlanta. However, the building in the background is the famous Bonaventure Hotel in Los Angeles, and an old RTD (now MTA) bus drives by.
The band's tour goes from the Eastern USA to the Western USA. The scene of the band discussing the "zodiac" dress style at an airport is supposed to be set with the band somewhere around the midwest USA (since it is supposedly two hours to Colorado from there and they are touring from west to east), but they are clearly at the LOS ANGELES airport in this scene (identifiably by the "Theme Building" visible outside the terminal windows.)
It may be an intentional fiction/obfuscation, but just after a shot showing the band as landing in Seattle, they perform at "Lindberg Air Force Base" due to a gig at the civic arena falling through. There is no such Air Force base. The only Air Force base in the Seattle area would be McChord, now part of Joint Base Lewis-McChord.
The limo driver talks about Sammy Davis coming out of the Copacabana about three in the morning and seeing Frank Sinatra walking down Broadway. The Copa was on 60th Street east of 5th Avenue -- more than half-a-mile from Broadway.
Marty Di Bergi wears two different US Navy caps, one in the film and one in 'Catching up with Marty Di Bergi' in the Special Features on the DVD release. In the film, the cap appears to say "USS Coral Sea OV-48". It should say "USS Coral Sea CV-43". The USS Coral Sea was an aircraft carrier in the US Navy from 1946-90, the second ship to bear that name. In the special features, the cap is from the USS Wadsworth FG-9, a guided missile frigate in the US Navy from 1978-2002. The Wadsworth was transferred to Poland in 2002 and renamed the General Tadeusz Kosciuszko.
During the airport scene when Derek Smalls, played by Harry Shearer, is told to empty his pockets and go back through the metal detector, it doesn't go off as he's walking back through to re-enter. A metal detector will go off no matter which direction one enters.
Before the Jazz Odyssey, David and Viv discuss the set list. David, running down the list says "Heavy, Hole." He's referring to two other Spinal Tap songs, "Heavy Duty" and "Hell Hole", in a sort of shorthand.
At the time, it was common for British rock stars to be "fake Americans". That would explain Nigel's pronunciation of "semi" and Derek's "zipper". Jeanine says "airplane" and not "aeroplane", and pronounces exit 'egg-sit' instead of the British 'ex-it'.
At the end, Russ Kunkel is credited as Eric "Stumpy Joe" Childs, the drummer who choked to death on vomit. However, Kunkel plays in the "Jamboreebop" video. Just after that, the band calls him "Peter 'James' Bond".
It's not shown in the film, but Peter "James" Bond was the drummer who exploded on stage.