It is thought that in the musical number "That's How You Know" during the mariachi scene, the trumpet players aren't playing due to the fact they aren't pushing down any valves. However any medium level trumpet player can pitch (use ones mouth instead of the valves) an octave. This band could easily be pitching the entire song.
Robert's secretary Sam admits she can't find the place that Giselle comes from - Andalusia, Andalasia, whatever. Although she's called every travel agent and every airline, she doesn't even know if it's a country or a city. Weirdly, it seems that no travel agent or airline has informed Sam that Andalusia (with its beautiful and historic capital Seville) is a large territory of sunny southern Spain, making this a major European tourist destination.
The dance at the end is the "King & Queen's Waltz", so the rhythm has to be 1-2-3, 1-2-3 etc., but it is actually a slower 1--2, 1--2 (or 1--2--3--4 if you want). Each actual beat COULD be imagined as a very quick 1-2-3, but nobody dances that fast. The dance is like a fox-trot, not a waltz.
When Giselle first wakes up in Robert's apartment, she sees the mess and starts her "Happy Working Song." In the song she sings nonchalantly about cleaning the hair from the shower drain. However, only a few minutes later in the bathroom, she expresses amazement to Robert about the existence of such a "magical room."
When Giselle takes a cup of water from the aquarium in Robert's office and spits it back into the cup, she spits out a male Siamese fighting fish (Betta splendens). A responsible aquarist would never have put that fish in with the aggressive cichlids and semi-aggressive gouramis in the tank.
When Nancy arrives at Robert's apartment to find Giselle there, Robert is in need of a shave. When he chases after Nancy as she leaves the building and gets into a taxi, he is clean-shaven. When he returns to the apartment, he has considerable beard growth again.
When Prince Edward reaches Robert's apartment to "rescue" Giselle, he picks Giselle up and spins her around. If you pay close attention, the first time he swings her around, Giselle's pink pumps fall off her feet. The second time, the shoes are back on her feet.
When Giselle first arrives at Robert's apartment, she gets her petticoat stuck in the door. She then falls asleep on his couch without the petticoat on. However, when she wakes up in the morning, she's wearing her petticoat again.
When Giselle is talking to Clara in the park, Clara's hands change position between shots. Then when Robert and Giselle walk away you can see Clara in the background, then she disappears, never to be seen again.
When Robert and Morgan find Giselle on the billboard, Morgan is wearing a karate gi and white belt. A karate belt is wrapped around the waist twice before the knot is tied. The knot is tied around both wraps of the belt in order to hold the belt together. During the billboard scene and the hallway scene after it, Morgan's belt is tied correctly. After Giselle falls through the doorway, Morgan's belt is now tied incorrectly, with the knot of the belt hanging below the first wrap.
In the Italian restaurant scene, there are two stacks of bread baskets behind Giselle's shoulder. Almost every shot shows a different bread basket configuration - baskets disappear and reappear. While one shot shows a waiter taking a basket, this cannot explain all the discrepancies. Maybe Robert isn't the only "magician" in the room!
Towards the end of "That's How You Know" in Central Park, Robert's phone rings while he's sitting in the carriage. In the long shot, he clearly reaches into his pocket and removes the phone. In the close-up, he does the same thing again.
Giselle and Robert leave the law office building. He says he can't help her anymore, gives her money and she walks away into Central Park. The next shot shows Robert turning his back ready to walk back to work. In the background, a man wearing black jacket and loose fitting jeans walks from the screen's bottom right to left. Robert turns around to take a last look at Giselle. The man now clearly walks out of the screen's left side. The shot switches back to Giselle. It then switches again back to Robert. This time however the man in the background is still in the middle of the screen when seconds before he had already walked out of to the left.
When Giselle returns to Morgan for help getting ready for the ball, she kneels down and her hair is partly over her shoulder and partly behind her back. When the camera cuts in close, her hair is entirely over her shoulder to the front.
When Giselle is knocking on the castle billboard, there is a camera shot from Robert's point of view. In the shot, one can see up part of Giselle's hoop skirt, under which are the dangling leather straps of her safety harness.
In the restaurant, a struggle breaks out between Nathaniel and Pip. Pip dives under the pizza and Morgan says "The pizza is breathing". When Nathaniel picks up the pizza to throw it in the oven, you can see the device on the bottom of the pizza that made it "breathe".
When Prince Edward is on top of the bus, he stabs the bus through the roof, and hits the old lady's bag, tearing it open. There's no way his sword was long enough to go through the roof of the bus and reach all the way to the woman sitting down. He'd have to be hanging upside down inside.
Nathaniel takes Edward to a motel, some restaurants, and other activities which require money or some other form of payment. Nathaniel also moonlights as a cab driver and waiter, abilities which one would not expect an "Andalasian" to pull off in New York. However, Queen Narissa seems to have some knowledge of New York, and could conceivably have arranged for Nathaniel to avoid trouble in these areas before he went on his mission.
It's said that during "That's How You Know", the heart-shaped flower wreath Giselle gives to the pigeons is different from the wreath that Nancy received. However, the wreath is simply seen from the other side in the scene with Nancy. The edge of the white flowers can be seen on the inner left side.
When Queen Narissa gives Nathaniel the poisoned apples in the soup, he reaches in to get them. His fingers touch the nearly boiling liquid (as evidenced by the bubbles rapidly rising to the surface) three times and he never so much as flinches.
In the scene where Edward comes to Robert and Morgan's apartment to get Giselle, as Giselle is explaining what a date is to Edward, if you look to the bottom left where Giselle was just standing next to Robert, you can see her pink "mark" tape on the floor.
The goof item below may give away important plot points.
When Nancy Tremaine sits down on the ballroom floor and picks up the single glass slipper she is dry even though she had been out on the balcony in the pouring rain looking up at the battle with the dragon.
Towards the end of the movie, when Giselle throws the sword to save Robert from falling off the building, the sword catches on the corner of his sleeve. When the sleeve finally gives, and he's falling and sliding down the roof of the building, his sleeve keeps switching how it's ripped between shots as he's falling. First only the cuff is ripped off, then none of it appears to be ripped, then the cuff again, then the whole sleeve is missing.