Continuity mistake: When Liesl creeps through Maria's window there's a close-up on Maria where the lighting on the wall is totally different and all the previous shadows are gone. A frame later, from a different angle, they're back.
Continuity mistake: When Mother Abbess tells Maria to leave the premises, the stuff on her desk changes positions.
Continuity mistake: During the storm, Gretel enters Maria's room and opens the door, which bounces back. A frame later the door is moving in the opposite direction.
Continuity mistake: In the Do-Re-Mi scene, when the children and Maria are running back and forth on the steps at the very end of the scene, when it is about to end, Kurt doesn't reach the top step, and after the song has ended, you see him stepping up to join everyone else.
Continuity mistake: When the kids fall off the boat, the Baroness's hand rests by her hip. A frame later it rests against the Captain's back.
Continuity mistake: After Liesel comes into Maria's room and Maria goes to look out the window, you can see the top of the ladder Liesel climbed up. In the next shot the ladder is gone.
Continuity mistake: During "So Long, Farewell", the large area rug in front of the kids moves just after the "cuckoo" part of the song.
Continuity mistake: During the "My favourite things" song the coat on the chair disappears.
Continuity mistake: When the Capt. and Maria dance, right before he starts clapping she is five meters away, but a frame later he stands barely a meter away.
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.).
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.
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.
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.