In addition to the continuity error about the anamorphic lens seen in the preview theatre two different versions of the rushes are shown in that scene. When Olivier and Vivien Leigh are watching they are looking at a very wide cinemascopic version but, when it cuts to a different angle as they stand up to leave, the image is more of a 16:9 ratio.
In the opening sequence, Marilyn appears singing "Heat Wave" in the screen of a movie theater. She sang that Irving Berlin song in There's No Business Like Show Business with a totally different set and costumes.
Greene tells Colin that he's known Marilyn for seven years. Greene and Marilyn met for the first time in 1953 when he shot her on assignment for Look Magazine, three years before the movie takes place.
A frustrated Olivier tells Colin that he should have cast Vivien to play Elsie instead of Marilyn. Marilyn bought the rights to "The Sleeping Prince" from its author Terence Rattigan, and hired Olivier, who agreed to co-produce the film, to direct; she could not be replaced.
In the screening room sequences, a close up of the projector lens reveals an anamorphic lens, which would be projecting the image in Cinemascope. "The Prince & The Showgirl" is not a wide-screen movie and rather shot in full academy, 1.37:1 aspect ratio. Furthermore, when the dailies are shown on screen, the image looks closer to 1.85:1, much wider than the original format of the movie.