Revealing mistake: When Mary Poppins goes to bring out the hat stand and the mirror, you can tell that the underside of the table is masked with a overlap of the background to make the illusion complete. Also, when she pulls out the mirror, watch the table leg closest to her, and the bottom of the mirror pass, because the illusion is not quite far enough to cover it.
Continuity mistake: When Admiral Boom makes his appearance, and asks about the time gun, he states "Three minutes and Six seconds" until the hour. However, the argument between Katie Nanna, the maid and cook, and Mrs. Banks' "Sister Suffragette" number, and Nanna's announcement of departure take a total of five minutes and ten seconds until the gun is fired.
Continuity mistake: When Mary Poppins is reading the childrens' torn-up note to Mr. Banks, she is wearing a pair of white gloves. The next shot is a close-up and she is wearing black gloves. And then finally a room shot, where she's wearing white gloves again.
Other mistake: When Admiral Boom orders the gun to fire on "the Hottentots" (the sweeps) the rockets sticking out are in the order of a small red on top, a large green in the middle, and a large red on the bottom. A moment later, he orders another salvo, but the rockets are in the same arrangement. The chances of the arrangement being identical twice, unless done carefully, are very slim.
Continuity mistake: When Bert is showing Jane and Michael his chalk sketches and doing his tightrope demonstration, you can see the pigeons in the corner on the far side of the bench. In the next shot when Jane points out the English countryside, the pigeons are right behind Bert again and walking away.
Continuity mistake: When Mary, Bert, and the children enter the tea parlor and see Uncle Albert floating by the ceiling, the camera cuts to Bert behind Mary, then Bert in front of Mary (Bert preparing to laugh and levitate), then a shot of the children shows Bert behind Mary again. Bert cannot move this fast in a split second.