Here's a few headings. I've started the lists.

KEY SPECIFIED BY SINGER. SONG THEN PLAYED IN DIFFERT KEY.

1. In the film - "Show Business" Eddie Cantor sings - "Curse of a Broken Heart"
Asks for it to be played in the key of E. Then it's played in D.

2. In the film - "Singin' in the Rain" Debbie Reynolds sings - "Singin' in the Rain".
Asks for it to be played in the key of Ab. Then it's played in E.


ACTOR PRETENDS TO PLAY INSTRUMENT. A DIFFERENT INSTRUMENT ON SOUNDTRACK.

1. In "The Fugitive Kind" Marlon Brando, in a boxcar, sings and plays a crap acoustic guitar. Electric guitar on soundtrack. Interesting point here is that Brando, portraying a professional musician in New Orleans, refers to the guitar as his "life's companion", and has carved the names of Leadbelly and Blind Lemon on its face. The guitar shown is one of those shallow plywood jobs with a tailpiece, that you could buy for $19.95 new. Leadbelly or Blind Lemon would have used it for kindling. Brando wears a snakeskin jacket that he could sell and buy a decent guitar, which any real guitarist would do.

2. Noted by Splott Man. In "High Society" Bing Crosby sings "True Love" while playing a concertina. Chromatic accordion on soundtrack. (Big chords with bass notes - not possible on concertina.)

3. In "The Human Comedy" Van Johnson gives a concert playing a large concertina. Chromatic accordion on soundtrack.

4. thespionage notes - In "The Muppet Movie" Kermmit the Frog playing wrong sort of Banjo.

And - The other side of the coin:
EXAMPLES OF GREAT LENGTHS TAKEN, BY MUSICIANS AND/OR DIRECTORS TO ACHIEVE AUTHENTICITY.

1. In "Horse Feathers" Groucho Marx plays a proper good quality guitar (very well) while floating in a boat. He's singing his own cynical version of "Everyone Says I Love You". Just before he throws the guitar in the river there's a subtle cut and the guitar is switched to become a cheap one.

A start, Cheers, Joy