
Revealing mistake: As Kevin is flying off the front porch on the toboggan, you can see the small wheels mounted on the bottom of the sled to aid in its "jump". (00:25:30)

Revealing mistake: At the beginning of the scene near the end of the movie with Lucius Malfoy fuming at Dumbledore in his office, Malfoy's hair is fanned back behind his shoulders. The lighting in the room illuminates the back of his neck, where you can see his real, short brown hair. (02:22:05)

Revealing mistake: When John picks his wife up at the airport, it's raining. But look at the people around outside, they are in T-shirts, in the sun, and not getting wet. Obviously the rain is being sprayed on the car.
tokke
Revealing mistake: When Darth Vader and Ben are fighting, Darth's chestplate is on backwards and the belt "buckle" is as well. It appears that the entire shot was reversed. (01:26:20 - 01:27:15)

Revealing mistake: When Anna and Hans are dancing in front of the lighthouse, we don't see the shadow of the railing or the ground they're standing on.
manthabeat

Revealing mistake: When all the kids are in the diner just before Chunk knocks over the water jug, Mikey starts to bang on the ground with a bar. When Brand stops him, Mikey calls him by his real name Josh. This is verified on the DVD by Sean Astin himself, in the commentary. (00:40:40)

Revealing mistake: In the final game against Iceland scene, there is a point where you can see that the people in the stands are actually cardboard cut-outs.

Revealing mistake: In the Forbidden Forest, HRH are introduced to Grawp. Just as the giant lifts Hermione off the ground, in the shot from behind her, Hermione's legs disappear below her calves; the digital lines are actually visible. Slow-mo is not necessary. (Only visible on fullscreen DVD.) (01:29:40)
Super Grover

Revealing mistake: When Troy says "I'll sing with her," Ms. Darbus turns around "in response" to his voice before he even says anything.

Revealing mistake: When Hallie trashes Annie's cabin and the huge water balloon is about to fall on Annie's head, the camera is at a downward view and Annie clearly steps into the trail of the balloon instead of jumping away. (00:18:05)

Revealing mistake: When Tallulah and the girls are acting in the Fat Sam's Grandslam song look at Tallulah as they sing the 'Ra da da da da da' bit. She looks straight into the camera.

Revealing mistake: When the Lion runs out of the Wizard's room, the group disappears because it is an obvious backdrop.
Sacha

Revealing mistake: When Tim Allen startles Santa Clause and he falls off of the roof, Santa's foot kicks up a blanket that was used to look like some snow on the roof. (00:14:10)

Revealing mistake: Just after Harry attacks Snape at the Shrieking Shack, he insists on knowing about Peter Pettigrew. In the next shot, as Lupin says, "He was at school with us. We thought he was our friend," there are wedges under the wheels of the piano, however, in the following shots they're gone. (01:34:10)
Super Grover

Revealing mistake: After Carmen rescues Juni, as they fly over the city she says, "Hang on butterfingers." In the following close-ups, where is the handcuff with the broken chain on Juni's wrist? He is not wearing it on either wrist in any of these close-ups. It's obviously back in the following shots. (00:41:45)
Super Grover

Revealing mistake: In the part in the motel where THING is doing morse code on the table to Gomez you can see the cut-out in the table where the guy's hand is sticking through the table.

Revealing mistake: When the kids step out of the water and walk towards the house, the floor is already soaking wet from previous takes.
Sacha

Revealing mistake: When Flynn and Maximus are fighting to get the satchel, it flies off and gets snagged on a tree branch. But you can see it goes through the branch, and not around it.
Ksla02Suggested correction: If you look closely, it is very clear that it doesn't go through the branch at all, but hangs from the side as it should.

Revealing mistake: At the beginning when the ruler is making his speech, you can see his modern day fillings in his upper molars.
drw60a
Revealing mistake: Aboard the Hogwarts Express, when Harry stands beside the open window and sends Hedwig off with the letter for Sirius, surely his hair and clothes would be wildly moving about, because of the strong wind from the fast moving train.