In the "A Pretty Girl is Like a Melody" sequence from The Great Ziegfeld, narrator Frank Sinatra says that Dennis Morgan is singing the song. In fact, Morgan's singing is dubbed by Allan Jones, not because Morgan's well-known tenor voice was unacceptable but because Jones had already pre-recorded the mammoth sequence. At the time, the still unknown Morgan was billed by his real name, Stanley Morner.
In the "Melody of Spring" sequence from Cynthia, narrator Elizabeth Taylor self-deprecatingly remarks that she "was certainly no threat to Judy Garland or Jane Powell." In fact, Taylor's singing was dubbed in the film, a point emphasized when she turns up ten minutes later in 'That's Entertainment!' with an entirely different voice in the "It's a Most Unusual Day" sequence from A Date with Judy. In this case, narrator Peter Lawford claims, "That isn't Elizabeth's voice you're hearing. MGM kept her too busy to rehearse and record."
During her narrative section, Liza Minnelli erroneously states that her mother, Judy Garland "once told me that MGM seemed obsessed with Shirley Temple. They even offered Fox Clark Gable and Jean Harlow just to obtain Temple for a picture Metro was preparing. But the deal fell through, so MGM went head with the picture and cast Momma in the role of Dorothy in The Wizard of Oz." In fact, Jean Harlow died before MGM began preparations for The Wizard of Oz.
At the beginning of the film, Frank Sinatra says The Hollywood Revue of 1929 is the "first all-talking, all-singing, all-dancing movie ever made,". In fact it wasn't, the first was The Broadway Melody, which was released in February, nine months before "The Hollywood Revue" was ever released. Indeed, by the time of That's Entertainment III (1994), narrator Gene Kelly was now calling The Hollywood Revue of 1929, "one of the first all-talking, all-singing, all-dancing movies."
Liza Minnelli states that she regularly rushed home from school to visit whichever set that her mother Judy Garland was working on at MGM. In fact, Judy Garland's final film at MGM was Summer Stock in 1949, when Ms Minnelli was all of 3 years old and, presumably in Kindergarten, unable to rush anywhere unaccompanied.