Continuity mistake: Liesl creeps through the window and explains herself to Maria, standing by the middle of the bed. When the angle changes she is by the side of the bed. Then the angle changes and she is back by the middle side.
Continuity mistake: Christopher Plummer throws down the drape the kids used for clothing on the ground. Then he walks away and there's a clear shot of the ground and the drape has vanished.
Continuity mistake: In the scene right before "Edelweiss", the Captain is asked by the children and Maria to play something on the guitar. The Captain is holding a glass in his right hand and then proceeds to raise his left hand in a negative gesture. The shot changes, and the glass is now in his left hand- with his right hand raised to gesture.
Continuity mistake: When Maria and the children are in a carriage riding around Salzburg, singing Do-Re-Mi, at one point (when she is sitting facing the kids) they pass a shop that is painted in a very distinct color and with a distinct sign (Frisor = a hairdresser) A few seconds later, as Maria turns around to sit forward in the carriage, they pass the same hairdresser.
Continuity mistake: During "Do-Re-Mi" Maria's hair is blowing in the wind in the long shots but is perfectly combed in the closeups.
Continuity mistake: Right after the boat tips over, the children change places between the first shot and the shot that shows them coming up out of the water. Most notably, Louisa moves from being over the left, in the front, to over the right and carrying Gretl out of the water. She doesn't have enough time to move back and pick up Gretl.
Continuity mistake: Nicholas Hammond (Friedrich) was not a natural blond. The first scene to be shot involving him was the "My Favourite Things" scene and you can see that his hair is pale auburn. After this scene was shot, the directors thought that he would look better if his hair were lighter, so they applied an early form of bleaching agent to his hair. In scenes which were shot after this (notably the children's first entrance to meet Maria which occurs before the "Favourite Things" scene) his hair is very, VERY blond, almost white. In fact, if you scrutinise his hair closely throughout the film, it is a different colour in practically every scene, depending on when the scene was shot and how much of the bleaching agent had been applied.
Continuity mistake: When the children are singing for the Baroness, the Captain joins them and enters the room. In this wide shot, we see Liesl only begin to put down the guitar. In the close up shot of the children, however, the guitar is almost all the way down.
Continuity mistake: During the Folk Festival, near the end of Do-Re-Mi, while they are singing the "with jams" Gretel is in the front row with Kurt directly behind her, but when it zooms out, Kurt is further along and Gretel is between Louisa and Kurt. (00:02:30)
Continuity mistake: The Captain sits on the middle of the couch to sing Edelweiss, but appears sitting on the far left side a frame later.
Continuity mistake: Part way through the "Do-Re-Mi" sequence, when Maria and the children are walking down the steps overlooking Salzburg, the sun changes direction. At first it's behind them, so their faces are in the shade. Then suddenly the light is in front of them, and their shadows have changed direction too.
Continuity mistake: In the scene right before the song "Sixteen Going On Seventeen", sung by Liesl and Maria, Liesl looks down and says a line ending with "...and he stops loving you." and then looks up. The shot from behind shows her looking up again. (02:25:45)
Continuity mistake: Just before the Captain is about to sing Eidelweiss to the children, Max, Maria and the Baroness, the Baroness says to Max "Well why didn't you tell me, I could have brought along my harmonica." She has a glass of champagne in her hand and is about to take a sip but the camera cuts back to the Captain starting to sing and the glass is back resting in her hand. (01:21:30)
Continuity mistake: After "How Do You Solve a Problem Like Maria?" a nun is shown walking Maria to Mother Abess's office. She says, "You may go in now, Maria", places her hand on Maria's shoulder, and then walks away. In the next shot that's viewing them from inside the office, the nun is seen repeating the same action.
Continuity mistake: After the boat overturns on the lake, and Maria is standing on the terrace arguing with the Captain, her wet hair style changes several times.
Continuity mistake: When the Captain is asked to allow the children to sing in the festival by Max, he protests. He crosses to a table and says "Max." and picks up a glass. The shot changes, he says "My children do not sing in public", and there is now a bottle in his hand.
Continuity mistake: At the party Captain von Trapp puts on for the baroness, there is a scene where the guests are being greeted by Captain von Trapp and the Baroness inside the doorway of the house. When Herr Zeller is greeted and he looks ahead you can see a lady with brown hair in a pale blue dress being greeted in the background and then looking over her left shoulder. It then cuts to a scene where Herr Zeller has moved further into the house away from Captain von Trapp and the Baroness, and you can see she is greeted a second time and again looks over her left shoulder. (01:25:05)
Continuity mistake: When Maria leaves the Abbey to go to the Von Trapps, she walks for a ways, then takes what appears to be a rather long bus ride. This shows a fair amount of distance between the Von Trapps' house and the Abbey. Yet, when Maria snuck out to return to the Abbey during the party, it was night, and she had no ride waiting. Later, after the Captain tells the children he will marry the Baroness, they walk to and from the Abbey in an unsuccessful attempt to see Maria, and in an apparently short amount of time. From the initial bus ride, the Abbey was simply too far away for those walks to be believable.
Continuity mistake: Maria goes to see Mother Abbess after they sing the song, when Maria enters the room the chair is at the end of the desk, but when Mother Abbess tells her to sit, it is clearly in the middle.
Continuity mistake: Maria sits in front of Mother Abbess's table, but when the angle changes she is several meters away.
Answer: Postulants/Novices, such as Maria, were forbidden from singing in the Abbey. However, these rules did not necessarily apply to the sisters who are singing. In addition, it's a musical, and the nuns weren't "really" singing. Their complaints about Maria are simply put into a song.