Continuity mistake: When the children and Baroness Schrader are playing the game with the ball, they all have numbers which they say as they bounce the ball to one another. These numbers then change once they start throwing the ball (Friedrich starts as 4 and becomes 5, the Baroness starts as 5 and becomes 6, etc.).
Factual error: When singing "Do-Re-Mi", Maria is playing a Goya guitar. Goya brand guitars were not invented until the mid-1950s, a good ten years after this takes place.
Continuity mistake: When the kids ask a nun about Maria, they stand apart in the angle from behind, but are closer together in the front one.
Continuity mistake: In the first scene where Maria is singing "The Sound of Music" there are two different camera shots; in the long shot the grass is a yellowy colour and in the close up the grass is suddenly green. (This has been fixed on the 40th anniversary DVD.)
Continuity mistake: After the "Do re mi " song the Capt.'s car makes a right turn and drives through a road. In the close-up it's several meters behind passing by the same trees again.
Continuity mistake: When Maria sings "and then I don't feel so bad", Marta's ponytail swaps positions for a brief moment.
Factual error: The story is set prior to the Anschluss, which was Austria's forced annexation with Germany in 1938. At this time Austrian vehicles still drove on the left (Hitler eventually changed that). When Von Trapp brings the Baroness to his home for the first time, he drives on the right.
Continuity mistake: When the Edelweiss song ends, Max starts to talk and the Baroness has nothing in her mouth. A frame later, from a different angle, she has her hand up holding a cigarette.
Continuity mistake: Mother Abbess is singing next to a window, but when the angle changes the window is gone and there's just a wall.
Continuity mistake: While Maria prays for the kids' new mother and Leisel enters through the window, the coat lying on the chair changes the way it's folded between angles.
Continuity mistake: During the boat scene, the baroness leaves and walks behind Maria. From the immediate opposite shot the baroness has vanished.
Continuity mistake: While the kids are watching the people waltzing, Gretl stands facing the room, but a frame later she's leaning sideways against the walls.
Continuity mistake: During the puppet show Marta changes the backdrop but from a different angle and for a very brief moment, it's the previous one, with a sky instead of a castle.
Continuity mistake: Before the Edelweiss song, the Baroness takes a glass to her lips, but a a frame later her hand is down.
Continuity mistake: When Rolf brings a telegram he extends his arm and immediately after, he lowers it. A frame later it's extended and he is lowering again.
Continuity mistake: While Liesel tells the nun that she wants to visit Maria, Friedrich and Kurt keep changing positions between shots.
Continuity mistake: When the ballroom dance starts, the Capt. and the Baroness greet the guests. The number of people walking down the stairs behind them increase, decrease or simply disappear between shots.
Factual error: Aside from the fact that split-level entries didn't exist in 17th century Austrian palaces, there would be no windows in main state rooms if you did enter the front door and step down.
Continuity mistake: While looking at the front of the house at night time, you see there is a window above the door, which is lit from behind. From the inside of the house, the front door doesn't have a window but has wainscoting above it.
Factual error: When they're making their escape in the Citrôen a shot is shown under the car. The tyres on the Citrôen are "Michelin X" radials. These were not on the market till 1948.
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.