Revealing mistake: When the kids step out of the water and walk towards the house, the floor is already soaking wet from previous takes.
Revealing mistake: In the scene where Captain von Trapp takes a Nazi flag and tears it in two pieces you can see him searching for the correct place to tear. If you look closely you will see that a small incision has been made already so it tears more easily.
Revealing mistake: When Gretl runs up to Uncle Max saying, "Can we really keep the puppet show, Uncle Max?", you can see Marta in the background begin her cue too soon as she starts to run up to Uncle Max in the middle of Gretl's line, but quickly stops herself. Then after Gretl finishes her line, Marta carries on and runs up to Uncle Max with the rest of the children and says her line.
Revealing mistake: In the Do-Re-Mi scene, when the children and Maria are running up and down the steps, Charmian Carr (Liesl) is doing the correct choreography, but was unsure of herself. Heather Menzies (Louisa) got it wrong, but did so confidently. Carr then takes a false step in order to make her movements match Menzies'. Carr confirms this in the 40th anniversary special: she second guessed herself during the filming, and that's why she corrected herself, but upon watching the movie, she realized she'd been correct in the first place. Viewers can confirm that Carr was correct because she initially lined up (diagonally) with the other children on her side, while Menzies was out of line.
Answer: In the film, the Mother Abess explains to Maria that the Captain Von Trapp's wife died "several years" ago. This is commonly misheard by viewers as "seven years." In reality, the actual mother of the Von Trapp children was Agathe Whitehead, who died of scarlet fever in 1922, just four years before Maria came to the Von Trapp home, initially as a tutor (not a governess) in 1926.
Michael Albert