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)
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.
Audio problem: After the credits, Bert makes a riddle about the cop and right after his accordion is heard, but he isn't playing it.
Audio problem: After Bert rescues the Fox from the dogs the Fox says "Tis an elegant merry go round horse." If you look closely you'll notice that the voice recording is slightly off-sync with the animation on the Fox's mouth.
Answer: Most likely a variation on the "inexhaustible bottle" magic trick which has existed in various forms for hundreds of years - it relies on different chambers holding different liquids, and air holes in the bottle being covered or not change the pressure and allow the desired liquid to flow out. As the scene is one continuous shot with the entire bottle visible, this seems most likely - the air holes could be on the side away from the camera, with Julie Andrews repositioning her thumb for each colour. More info here: https://en.wikipedia.org/wiki/Inexhaustible_bottle.
Jon Sandys ★