There is a deleted subplot in the movie which explains the cold sores on the band members' lips: the band takes on an opening act for the tour and the lead singer sleeps with each bandmember, giving each one herpes in turn.
Harry Shearer, Rob Reiner, Christopher Guest, and Michael McKean were given $10,000 to write a script. A 20-minute version of the film was made with the money to better demonstrate the improvisation they had in mind. Several scenes from this demo are in the final movie.
It is revealed that "37 different people have been in the band over the years". Minus the two original members, one keyboard player, and the original and current bass players. This implies that the band has had 32 different drummers who inexplicably died.
Tony Hendra (who plays manager Ian Faith) writes in his memoir "Father Joe" that he attempted suicide the night before the first day of filming. He credits the joy he experienced in making the film with bringing him back from his depression.
In the course of the film, Spinal Tap has four different drummers: John "Stumpy" Pepys, Eric "Stumpy Joe" Childs, Mick Shrimpton, and Joe "Mama" Besser. These names were inspired by the four different men who played the role of third Stooge in The Three Stooges films: Curly Howard, Curly Joe DeRita, Shemp Howard, and Joe Besser.
Before the first song of the film, an announcer introduces the band with this: "Ladies and gentlemen, direct from Hell, Spinal Tap." This is a play on Venom's intro tape from the early 1980s, which went: Now, from the very depths of Hell...Venom!"
As the film was improvised by all the performers, Rob Reiner, Christopher Guest, Michael McKean, and Harry Shearer went to the Writers' Guild hoping to give proper credit to everyone. The Board of Directors voted 15 to none that the credits should stay as it was including only the four of them.
There's a common misconception that the "too small Stonehenge" disaster is a parody of Black Sabbath's oversized Stonehenge sets from the Born Again tour. This is impossible, the Stonehenge Spinal Tap scene existed as early as 1982 when the film existed as a 20-minute short. Black Sabbath didn't begin using their Stonehenge sets until 1983.
There is no actual "Isle of Lucy" in the United Kingdom off England's coast, where Spinal Tap supposedly played a blues/jazz or jazz/blues festival. In one of the most subtle and overlooked gags of the film, they are really just paying homage to the classic television show, I Love Lucy.
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 read USS Coral Sea OV-48. This should be USS Coral Sea CV-43. The USS Coral Sea was an aircraft carrier in the US Navy 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.
Nigel's line about being able to "go out and get a bite" and "you'll still be hearing that one" when describing the sustain on his guitar is a paraphrase of Les Paul's description of the sustain on one of his own guitars, "You could go out and eat and come back and the note would still be sounding." The guitar Nigel is describing is a Les Paul.
Spinal Tap is infamous for things going wrong with their Stonehenge props. The most famous incident comes from the film, in which the prop is undersized and nearly trampled by a dwarf. On their live tour in support of Break Like The Wind, a package delivery man brings a package with an even smaller model. In The Return of Spinal Tap, the prop is far too large, and the stage crew makes every effort to cram it through the small doorway - unsuccessfully. When performing at Live Aid, the prop (signed by all the other performers) was the right size, but a timing error ruined the effect - the "columns" were lowered without the top crossing piece, and subsequently removed from the stage - only to have the top piece eventually lowered with nothing to land on.
Nigel talks with DiBergi about being influenced by the masters, including Wolfgang Amadeus Mozart and Johann Sebastian Bach. During their performance of "Heavy Duty", Nigel's solo (which he plays while standing on the drum riser) is a tribute to Luigi Boccherini's "Minuet from String Quintet in E major, G.275".
In Norway "This is Spinal Tap" opened directly to video over two years later. The title was changed to "Help! We are in the Pop Business!" (="Hjelp! Vi er i popbransjen!") This is a spin on the Norwegian title for "Airplane!" (1980), which was "Help! We are Flying!" (="Hjelp! Vi flyr!"). The poster didn't show any images of the band. Instead it displaying a guitar with a knot on it, similar just like the airplane on the "Airplane!" poster. Throughout the film there is a disclaimer on screen informing us that this was not a real band, this was all fake.