Deliberate mistake: When the bear pulls Woody into the garbage bin after the other toys have crossed over, his hat is not pulled off when his body is squeezed under the lid when going down. (01:15:35)
Deliberate mistake: When "Legs" lowers the Pez duck named Ducky down through the removed lightbulb hole, to ring the doorbell, the lightbulb housing is bigger than the hole but they bring it up through the hole to remove it. (01:03:55)
Deliberate mistake: In the scene where Emmet drives to work, you get Emmet's view of the dashboard with his hand switching on the radio. The car at that moment has no front window, perhaps to make the car look bigger from the inside. (00:04:50)
Deliberate mistake: Bowser has several teeth that appear and disappear throughout the movie. The best example of this is when Mario is trying to grab the Power Star. We see Bowser snarl at Mario before he picks him up. There is a gap between his two front teeth. However, when the scene cuts from Mario's horrified face to Bowser's face, there are now extra teeth inside that gap.
Deliberate mistake: When Tarzan fights with the leopard, it scratches his chest. However after they fall into the pit and Tarzan comes out carrying the dead leopard, the scratches on his chest have amazingly healed. [It is a long-standing Disney tradition that no evidence of violence is shown on screen. Still a mistake, but there's why].
Deliberate mistake: At the elephant graveyard, little Simba claws Shenzi in the face. When Mufasa saves Simba from the hyenas, the cuts are gone, a few seconds later. [This was done intentionally, I'm sure, for two reasons. The first being, Disney doesn't let fresh scars linger on the screen too long. Secondly, during that same scene when Mufasa rescues Simba & Nala, he claws Banzai in the rear. To stress the point later in the movie, that severe scar is shown too, but only briefly, it too disappears because of reason 1. Still a "mistake", but there's why.] (00:21:50)
Deliberate mistake: Towards the end of the movie fish are seen to swim down in order to submerge the net in which they have been caught. Although the attempt seems logical, it actually needs power against the net to push it down. None of the fish seem to touch the net at the bottom however and if it is the whole mass of fish achieving it, then the bottom fish should have been squashed against the net which (again) isn't the case. 'What brings the net down?' is the question here. (Done deliberately by Disney/Pixar to be visually aesthetically pleasing.)
Deliberate mistake: When Boo laughs repeatedly while playing hide-n-seek with Sully in the men's room, nothing happens to the electricity - no surges as with all other times she laughs. [In the audio commentary by the directors, they explained that it would've been distracting if an energy surge occurred every time Boo laughed, so they simply didn't address the issue. A reasonable course of action, but still a mistake.] (00:37:30)
Deliberate mistake: The morning after the flyers are spread, Buster is sleeping in a drawer. When his alarm clock goes off, he hits his head at the top of the drawer, but later on, he can be seen standing up in it, easily visible when he is putting his coat on. The drawer obviously became wider for him to stand up in it.
Deliberate mistake: During the fire Marge runs back into the house and goes to the kitchen cabinet that has a "keepsake cabinet" nameplate and a keyhole, in order to remove the wedding video, but the nameplate and keyhole didn't exist before. (00:33:35)
Deliberate mistake: When the Looney Tunes are playing basketball against the aliens, if you look in the stands the same cartoon characters are in different places at the same time. This happens a lot.
Deliberate mistake: Manny refers to the pirate rabbit as the Easter Bunny, even though Easter never existed in prehistoric times. (00:33:55)
Suggested correction: And mammoths didn't hang out with sloths and smilodons either. It's creative license.
Deliberate mistake: At the end of the film, Gaston shoots the Beast with an arrow and pushes the Beast out the window. When the Beast lands on the edge of the castle rooftop, the arrow has disappeared. Even if part of the arrow has broken off, a piece of it would still be in his back. [This, like many other incidents in the film, is a longtime Disney animated tradition. The company seems to believe that violence is less unpleasant if all evidence of it disappears quickly. This is why so many cuts seem to disappear or become less serious. The arrow vanishing is just another example.] (01:16:55)
Deliberate mistake: The 'NNN' news program shows a picture of some animal crackers in Frank's stomach. The problem is, they are still all in one piece and are pretty big. I don't think Frank would swallow them whole.
Deliberate mistake: Main headlines in newspapers appear translated for each country for the kids to understand it, but the rest of the news appear in English, despite the movie taking place in Paris. Also, most signs -such as the poisons in the window shop- have labels written in English.
Suggested correction: By that logic, it's a mistake that the characters speak English. It's an American movie set in France, and like you said, it's a kids' movie. Everything is in English for simplicity's sake.
Deliberate mistake: During the movie, when Mulan is just a girl, you can see that her eyelashes extend beyond her eyes. When she is pretending to be a boy, her eyelashes are short (this makes perfect sense). But when she is in the medical tent her eyelashes are long even though she had no time to put on makeup. This is deliberate though, because without it she would have still looked like a boy, and just pulling her hair down wouldn't make her look any different (since that's how she looked in the pond). So they had to change her eyelashes to make her look more girlish.
Deliberate mistake: The balloon llama the chipmunk makes has legs which are completely detached. The pairs of legs should be twisted together. Without this, it looks like the balloon has 6 different ends.
Deliberate mistake: You might be wondering where Terry suddenly comes up with Jolly Jack all of the sudden after escaping the lab. In the commentary, they say that the Joker, in his transmission to Terry in the lab, was supposed to be eating out of a candy jar with "Jolly Jack" on it, but couldn't have it due to time constraints.
Deliberate mistake: The 100-meter 'dash' goes over a longer distance and lasts longer than shown in the movie.
Deliberate mistake: After Baloo and Bagheera escape from the monkeys, they both have a black eye. However, during their conversation by the water, between shots their black eyes vanish. [This is a Disney tradition, but still a mistake.]