Revealing mistake: When all four actors are up on the rooftops, look closely at them when they all step onto the smoke platform that brings them down. All four actors are doubles. (01:46:50)
Revealing mistake: An early scene with the constable and the children, Jane silently mouths along with one of Michael's lines.
Revealing mistake: In the scene where Mary Poppins is removing the large plant from her carpet bag, Michael inspects the underside of the table, where the table leg casts a shadow across his face. When Mary crosses in front of him (still under the table) she does NOT cast a shadow, although she should.
Revealing mistake: In the tea party on the ceiling sequence, 2 shots after Uncle Albert tells his long underwear joke, Jane does a backflip in the air. If you look closely you can see the imprints that wires that she was hanging from made on her dress. (01:15:55)
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.
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.