Deliberate mistake: To make Jim Braddock look more heroic, Max Baer is portrayed as being very mean and hateful. In fact, the real Max Baer was an extremely pleasant man who was devastated when Frankie Campbell died after fighting him. Many boxing historians and sportswriters have complained about this inaccurate depiction of Baer.
Deliberate mistake: When the Matoban delegation arrives at the UN, it shows the motorcade entering the circle in front of the UN Secretariat building and stopping at the revolving doors right at the front the staff entrance. Diplomats' motorcades always continue on to the delegates' entrance under a protective tent in front of the GA building so as to allow VIPs to alight from their vehicles to prevent potential snipers from having a view of them. The very next shot of the ambassador shows him riding the escalator, which is at the delegates' entrance, going up to the General Assembly building. This would not have been possible unless they made him walk about 300 meters outside in broad daylight. No chance of that seeing he was already being protested on 2nd avenue all the way from the Queensboro bridge. Obvious reason for this error: The shot of the delegation getting out of the cars looks far better in the open right in front of the UN secretariat building rather than inside a protective tent.
Deliberate mistake: This was probably done intentionally to make them more visible, but the bullet holes left in the stairwell wall by Bart's handgun while he is chasing Danny during the movie's climax look to be about the size of golf balls. Typical handgun calibers aside, just looking at the size of the clip he's loading into the gun and seeing how many rounds it holds, those rounds would have to be much smaller than the holes the bullets made.
Suggested correction: It's a 9mm Glock18. The standard magazines have 17 rounds, the long ones 30. The holes could be larger because it's an old building and the plaster might break apart differently than modern drywall.
Deliberate mistake: The tiny hot air balloon that Francesca released in the debate room would never be able to support Casanova's weight.
Deliberate mistake: In the beginning, the police chief says "The six members of the family were found last night, in bed, shot and killed." As he says this, look to the left of the screen. In the background, the real Amityville house (dark brown with white windows and trim) is shown instead of the house used in the rest of the film. That segment was from actual news footage of the murders from 1974; apparently, the director didn't want to recreate it.
Deliberate mistake: During the dance number in the Dead Bar, the blue pirate has a sword stuck through him with the handle in the front, later it changes to the back, then to the front again. Seen when the groom falls back and grabs the handle, then picks the pirate up and drops him. (00:19:15 - 00:20:05)
Deliberate mistake: As the horse named "Sir Trenton", who is supposed to be a stallion, is galloping away you can see that "he" is a female.
Deliberate mistake: The mystery begins as Winston points out the postmark on the envelope that Dan receives, it is too faint to see where it's from. But whenever we see a close up of the envelope, there is no postmark at all.
Deliberate mistake: It's Christmas in Denver and there is snow on the ground. However, there still are quite a fair amount of leaves on the trees in various shots.
Deliberate mistake: When Constantine comes out of the library after speaking with Gabriel, he stands beside Angela and none of them cast any noticeable shadows on the door and walls behind them. When it cuts to a shot above them, shadows are more being cast behind them. This is probably because of different light settings on the set. (00:23:35)
Deliberate mistake: In the ambulance scenes, John Goodman is given oxygen via a nasal tube, probably so he could tell his story. In real life, a mask covering the nose and mouth would be used for maximum effectiveness in such a critical situation.