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 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.
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.
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 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)
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: 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 Troy says "I'll sing with her," Ms. Darbus turns around "in response" to his voice before he even says anything.
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 kids step out of the water and walk towards the house, the floor is already soaking wet from previous takes.
Revealing mistake: When the Lion runs out of the Wizard's room, the group disappears because it is an obvious backdrop.
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)
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)
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: In the scene where Doc Brown is following Biff into the tunnel, as Marty starts to pass between the cars on his hoverboard, one of the wheels on the rig supporting the hovering DeLorean is visible. (01:30:30)
Revealing mistake: When Woody falls out of the closet wrapped in the string lights he hears Buzz's voice, and in the next shot as Woody sneaks up to Hannah's room we see Sid's open door and doorway, which looks perfectly normal. However, after Woody rescues Buzz from Hannah's tea party they both stop in the hallway, and there's another shot of Sid's open door and doorway, only this time it's not normal to say the least. Five of the six hasps, bolts, and locks that should be (and normally are) screwed into and attached to the inside of Sid's door (one hasp mechanism, third one down from the top, is attached to the door's frame) are now actually kind of floating there in mid-air right beside the doorway, as if there's an invisible door there. Weird that Pixar artists deliberately did this, but it's bizarrely funny to see.
Revealing mistake: Mrs. Doubtfire sets herself on fire. When she jumps back and starts fighting the flames, it is obviously a stunt double as her face is longer.
Suggested correction: If you look carefully, the face looks pretty much the same, as well as the voice.
The reason the voice was the same is that it was probably an audio dub, which is a voice recording over someone else's voice.