Continuity mistake: When everyone is on top of the roof, Mary Poppins powders her nose, which results in her nose being black with soot. The scenes following that, her nose is normal and never black as when she powders it.
Revealing mistake: When the penguins walk in front of the table where Mary Poppins and Bert are sitting, the table can still be seen through the penguins. (00:48:30)
Factual error: At the end of the scene where Bert is a one-man-band, he smashes a cymbal on his forehead at the end of his song. The cymbal could not make any sound as he is holding it with both hands, preventing any vibration.
Continuity mistake: When Mary Poppins just starts to sing "Feed the Birds", she sings "The little old bird woman comes" but if you look very closely at the end of this shot, you can see that her mouth does not finish the word "comes" (her mouth stops at come).
Factual error: The local police constable wears the divisional letter 'H'. H Division of the Metropolitan Police covered Whitechapel. While it is never specified where the upper middle-class Cherry Tree Lane is, it's certainly not in Whitechapel, which was a solidly working-class district in the heart of the East End. He also wears his letter and number on his helmet plate, a 19th century practice that had been abolished by 1910.
Revealing mistake: During the 'Spoonful of Sugar' musical number, when Mary Poppins is singing to the robin in the children's room, the wire connected to the audio-animatronic bird can be seen just under Poppins' two fingers, very close to her palm.
Continuity mistake: Bert makes a riddle about a short woman and her daughters behind laugh, lowering their heads and turning them around. A frame later, from a wider angle, they are repeating the previous movements.
Revealing mistake: After the credits all the smoke coming out of the chimneys remains still, revealing it's a painted set.
Continuity mistake: In the shot where Jane and Michael are looking out the window at Mary Poppins you can see Jane's fringe being blown to the right side of her forehead because of the wind. In the next shot they're combed perfectly straight again.
Continuity mistake: During the credits Mary puts make-up on her face and raises her umbrella and bag. When the angle changes they both disappear.
Revealing mistake: When Mary Poppins has the robin on her finger, just after the outside shot where the robin does his whistle bit, you can see the cables used to operate the bird on the back of her hand while she turns away from the window.
Continuity mistake: During the first few scenes, you see Jane is missing a tooth but the tooth disappears and reappears throughout the film.
Continuity mistake: When Mr. Banks takes the children to the bank, and they meet the directors, first the children are standing close together, with Mr. Banks right behind them. Later, when the elder Mr. Doors is talking to them, Mr. Banks is next to Michael and the children are farther apart. Later still, when we see Michael close up, Jane is out of the shot altogether, meaning that she is even farther away from him than previously.
Revealing mistake: When the rooftop dance sequence begins and 3 of Bert's pals appear to shoot out of 3 round smoke stacks, they are actually coming from behind the stacks. (01:47:35)
Continuity mistake: When Mary Poppins ascends to join the others for tea at Uncle Albert's house, she starts with her purse clasped in her hand. As she ascends, the purse is suddenly draped over her arm.
Continuity mistake: Bert introduces the Admiral, who stands by the end of the banister, next to the canon. A frame later he has moved about 5 meters to the left.
Continuity mistake: Jane and Michael switch sides while they are holding Burt's hand when he walks them home.
Continuity mistake: When the kids meet Mary, Michael is in awe standing against a door. A frame later his head rests half on the door and half on its white frame.
Revealing mistake: Mary takes a huge mirror out of her bag and the kids stare in awe. Problem is that Michael's view points in a different direction, giving away that they were looking at the void and Mary was a superimposed image to achieve the effect.
Continuity mistake: When the bank executives are basically harassing Michael to give them his money, there is a shot where they are singing and marching forward and force the children to back into the wall. There is a pattern of large squares on the floor, and the men in the back are on the inside edge of the square. Then the camera goes out for a long shot so Mr. Dawes can talk about the banks of England. Now he is standing on the edge of the square. When he finishes his little speech, in the next shot they are standing back in the original position.
Chosen answer: They recorded her singing in two separate parts and fixed them together. The editors did a good job on it! It may have been made a long time ago but even then the technology was quite advanced enough to carry out that scene.