Revealing mistake: When Yoda knocks Darth Sidious backwards over his chair, you can see the stuntman's hand. Sidious's hands are old and his nails are dark around the cuticles, but the hand during the flip is perfectly healthy and a different size. (01:45:35)
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.
Revealing mistake: When the Hudson Ferry captain casts off the large label that reads "Hudson Ferry" is on his left side of his jacket. However, in the exterior shot as he looks out the window at the whirlpool, the label is on his right side. This shot was flipped, and amusingly, the CGI guys actually retouched the words "Hudson Ferry" so it wouldn't read backwards. (01:02:25)
Revealing mistake: Near the end of the film, when people are running in terror out of the theater and into Times Square, you can see that the people are not leaving any footprints in the snow. The snow must have been digitally added to the scene.
Revealing mistake: When Tom joins Sarah and Eliot for the kneeboarding, after Sarah apologizes for Tom's interrogation Eliot says, "It's okay, my dad would've done the same thing." Two shots later, as Tom grunts in the water, this shot is flipped - the driver is now on the left and the kids switched sides.
Revealing mistake: When the Muslim emissary arrives to Jerusalem and Guy de Lusignan stabs him under the chin, look at the blood. It bursts from something attached to his shoulder. In addition, the emissary is seen as stabbed with a small dagger but then Guy De Lusignan is shown holding a full length blood tipped sword a second later.
Revealing mistake: When Jimmy and War are fighting over the gun near the end of the movie, you can see that many shots are reversed, as the fire behind them is not burning upwards like regular fire, but appears to be "sucking" downwards, back into the barrels.
Revealing mistake: When Augustus is leaving the factory, he is wearing a brown colored glove on the hand he gnaws on. You can see the seams in the glove.
Revealing mistake: When the girls are on Aslan's back, you can tell that there is a flat screen behind them.
Revealing mistake: Near the end of the scene, when Bruce and Fox are in the tumbler Fox says "We never could get the damn bridge to work, but this baby works just fine" then Bruce turns the wheel to the left but the tumbler turns right. (00:57:45)
Revealing mistake: In driving rain, the faces of Nick and Suzanne appear to be completely dry.
Revealing mistake: The "metal" axe in his hand bends as Jacob flies through the window of the tower and smashes into the floor.
Revealing mistake: When Lavagirl burns the diary, it goes up in flames. However, if you watch Max and Sharkboy, they look at an unseen object falling to the ground. The unseen object is the diary, taken out in editing. Later, Max looks back at the diary on the ground.
Revealing mistake: When Elektra attempts to resurrect dead Abby, you can see Abby's chest rising and falling.
Revealing mistake: In the scene where Stripes is looking in the water at his reflection, his body and the reflection don't quite match up. You can tell by his movements that his reflection is computer generated. (This is verified by the audio commentary.)
Revealing mistake: Aeon's neck freckle swaps sides in one shot, revealing a flipped shot.
Revealing mistake: When Will flies back in through the window and picks up Royal Pain, you can see that the back of Will's jacket is sticking up, which was where the harness was lifting him up. (01:24:05)
Revealing mistake: On Miranda there is a shuttle crashed into the side of a building, but there are no burn marks or a trail from where it had crash landed. The shuttle couldn't possibly have crashed into the building horizontally without crashing diagonally/vertically first, landing horizontally and then sliding into the building. Joss Whedon even states in the director's commentary that some burn marks would have been nice but it only occurred to him about 14 months after it was filmed. (01:17:15)
Revealing mistake: When Al is running from the fire in the incineration chamber, fire is shooting out the back. When the fire goes away lights can still be seen in the chamber entrance. The fire should have incinerated them, because this area would be sealed off from the hot area during normal operation.
Revealing mistake: During the shoot out with McGivens's gang at the Cortez ranch, the wife of Cortez sneaks out of the trap door with her baby and peers around the side of the building. You can notice that the baby is really a doll.