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: 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: 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: When Arthur is drawing letters of the alphabet, he only draws the outside of the letter A, not the horizontal stroke in the middle, yet it appears on the blackboard.
Deliberate mistake: In the famous sequence where the pumpkin turns into a carriage, the fairy godmother stands behind it waving her wand and doing the magic. First mistake: After the transformation, the fairy godmother is stuck in freeze frame with her hands up in the air for the entire time when Cinderella is saying how lovely the carriage is. Another mistake occurs when it cuts to the fairy godmother (after the freeze frame), because her arms are suddenly down and she is moving and talking to Cinderella.
Deliberate mistake: While the mouse is walking on Dumbo's trunk, there is a jump in the animation. He's facing Dumbo at one point, then all of the sudden he is in a different position and is facing the other way (this all occurring in the same shot). Look for it before the mouse says, "Dumbo, the great!"
Deliberate mistake: When Penny is loaded onto the ambulance at the end, her gurney is rolled straight in. This is wrong, the paramedics will collapse the wheels and slide the gurney into locks to hold it still during the drive. It was done this way since Mittens and Rhino were hiding in the collapsible section.
Deliberate mistake: I know this has been disregarded for the fun effect, but gasoline motors require air to work. When Edgar drives his tricycle into the river while being chased by Napoleon and Lafayette, it should have stalled and stopped working.
Deliberate mistake: When the Darlings discover the dead rat, they raise the curtain and cry out loud, but the rat is nowhere to be seen. Given Disney's long time history of refusals to show dead bodies or blood, it's highly possible that this was a deliberate mistake, but really awkward indeed.