Character mistake: In the beginning of the film, Oliver has been brought by Mr. and Mrs. Bumble to the Governor's door after asking for more to eat. Mrs. Bumble then sings, "They'll lay the blame on the one who named him," and as Mr. Bumble is responding, "O-li-ver", look at Oliver. He is completely spacing out and not in character at all - staring off into the distance and flicking his tongue in and out of his mouth.
Character mistake: When Arthur Weasley takes Harry aside in The Leaky Cauldron to warn him about Sirius Black, he says, "13 years ago, when you stopped.... You-Know-Who...Black lost everything." Harry is 13 yrs. old in Prisoner of Azkaban and was a 1 yr. old when he stopped Voldemort. Hence, Harry stopped Voldemort 12 yrs. ago, and not 13! Harry turns 11 just before school starts in Philosopher's Stone, turns 12 before school in Chamber of Secrets and turns 13 yrs. old before school in Prisoner of Azkaban. Just an aside, the last digit of Harry's age always coincides with Harry's year at Hogwarts. So, he's 13 in his 3rd year at Hogwarts, which is entitled Prisoner of Azkaban. (00:17:50)
Character mistake: In the scene just before Harry and Cho kiss in the Room of Requirement, there is a clear Daily Prophet headline to the right of the mirror - 'Mystery Disappearences' a mis-spelling of the word disappearances.
Character mistake: After the Lieutenant is shot and is in hospital, his CO is explaining what the GHOST system is. He explains that the acronym is "Guided High Altitude Scrambling Transmitter" - which would be GHAST. Replacing "altitude" for "orbit", which is probably what was intended, would make a Guided High Orbit Scrambling Transmitter - or the GHOST system henceforth referred to in the film. (00:06:25)
Character mistake: When Jimmy explains to the kids of the town where the aliens took their parents, he mentions the origin of the aliens being somewhere in the Orion Star System, 3 million light years away. The Orion system is way closer than that. It is actually less than 1400 light years away from Earth. 3 million light years away would place them further than the Andromeda Galaxy.
Character mistake: During the race sign up scene, when each racer is throwing their coin into the gold cup as the entry fee, the announcer says Snowanna Rainbeau, and you can see a blue haired pig-tailed character just behind Rancis Fluggerbutter. This character is Citrusella Flugpucker, the 6th listed racer on the board, but she is announced as Jubileena Bing-Bing, who is listed 13th on the board. Jubileena Bing-Bing is the red pig-tailed racer, not the blue. (00:35:10)
Character mistake: At the beginning Sally follows Jack at the end of the Halloween celebration where he sings "Jack's Lament." She then goes off to gather more deadly nightshade. Look at Sally when she sits down. It is in front of a stone labeled "witch hazel" and "deadly nightshade" is seen next to it on the right. Yet, Sally pulls two pieces of witch hazel out of the ground, before a quick cut back to the lab where we see her putting it in a jar marked "nightshade".
Character mistake: When they deliver Xmas presents to Denmark it's already too late - Denmark has the presents on the 24th, early in the evening.
Character mistake: When Chessy confronts "Annie" when she's presenting her with the big breakfast, she's talking about when Annie was on the phone the night before. Then she says her father overslept, when she actually meant to say and "and you overslept," as the conversation doesn't make any sense saying that the father overslept. (00:58:00)
Character mistake: In her speech outside the bank, Mary Lou Barebone mentions "the wireless" as one of the wonders of modern technology, which is the British term. A New Yorker in 1926 would more likely use the Americanism "radio."
Character mistake: The closing shot of a newspaper article shows the word "rumor", film is set in England where it's spelled "rumour".
Character mistake: When Boris and Natasha (Jason Alexander and Renee Russo) are at the airport (just missing Rocky, Bullwinkle and the FBI agent) Natasha says something about "Moose and Squirrel don't even know our names", yet earlier in the courthouse scene, Bullwinkle says something about Boris and Natasha being real people and Rocky and Bullwinkle still being cartoons.
Character mistake: In the scene where the dentist is doing a root canal, Bloat says "rubber dam and clamp installed?" and another fish replies "yes", but when the picture goes back to the dentist and patient you can plainly see that there is not a rubber dam on the patient. A rubber dam is a large piece of latex attached to a metal frame, to keep water from the dental handpiece from going into the patient's mouth. Although all of the dental jargon in the film is precisely accurate, this device would have been very noticeable.