Revealing mistake: In the tea party on the ceiling scene when Mary takes out her watch, watch closely, she doesn't take it from behind her jacket. When she seems to reach behind her jacket it just appears in her hand. When she puts it back she reaches behind her jacket again, but it just disappears just before she puts her hand back there.
Revealing mistake: In the scene with the queue of Nannies, and the chimneytop scenes, the wires holding the actors can be seen clearly.
Audio problem: During the Spoonful of Sugar song Mary stands in front of a mirror and sings along with her reflection. Her reflection sings "they find." but the mouth movements do not match. (00:33:30)
Revealing mistake: When Mary is taking the things out of her bag when she arrives, she takes out a large floor lamp. You can see the lamp's post coming up through the bag and the table as she pulls it out.
Audio problem: In the French, German and Spanish version, in the song Supercalifragilisticexpialidocious, before Mary Poppins says the word backward. While Mary and Bert sing in the dubbed version, in these three languages, you can hear the English version in the background. You can hear it louder in the French version.
Continuity mistake: When Mary Poppins starts to read the "qualifications", the paper she reads from is whole, with no kind of indication that it had ever been torn. After a quick cut to George and back to the paper, the closeup of the paper reveals the lines where it was torn. Also in this scene, when she comes in, she has white gloves on. In the closeup on the paper, the gloves are gray, and a few shots later, they're white again.
Other mistake: After Bert and the others dancing on a railing that should have moved, as it did when they jumped off, three sweeps walk back and forth on their hands over the street between buildings. After dropping to only their toes to hold them up, you see them drop all the way down, then stand, where there was no surface in sight to do that on.