Revealing mistake: In the scene where Nanny McPhee seems to suddenly "appear" to Cook, after the children have gotten sick, you can see Nanny stand up from behind the counter, if you watch closely through the red bottle as the camera passes. (00:34:05)
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: When Kadaj kneels in front of Rufus Shinra, watch his sword. As he kneels, it goes through the floor.
Revealing mistake: Towards the end when Harmony calls Perry's phone as she's lying on the ground, Harry grabs the phone and looks at the caller ID to see her name. Unfortunately that's not the display showing who's calling. Anyone who has a Nokia phone will recognise that that's showing the contact list with just two entries - Harmony and a random number. The fact that it says "contacts" at the top of the screen is a bit of a giveaway.
Revealing mistake: When Stevie's car is hit by the delivery truck, the side shot that shows the car beginning to roll reveals there is no engine in the car, as we can see through the front fender wells to the truck as it passes on the far side of the car. (01:15:40)
Revealing mistake: During the garage fight scene towards the end, we see that the steel pole that Frank uses to beat up the bad guys bends while he uses it.
Revealing mistake: About a minute into the movie we see Ethan Hawke waking up. He has a tattoo on his left pectoral. He then goes to the sink and mirror and the camera is directly behind him. He drops out of shot and in the same shot reappears in "the reflection" as he splashes his face with water. His tattoo is still on his left pectoral - all he's done is drop out of shot, move forward a few paces, turns around towards the camera and reappear as the mirror reflection. They've forgotten to either flip the image around or put the tattoo on the other side. (00:06:00)
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: When Dekker follows Richards on a motorbike he drives through a glass door with wooden frame. The class shatters noticeably (about a half second) before the bike even touches the door.
Revealing mistake: When Gretchen turns around while on the bed, a shard of glass in the frame flaps in the wind.
Revealing mistake: Aeon's neck freckle swaps sides in one shot, revealing a flipped shot.
Revealing mistake: When dead Jackie Boy is talking to Dwight, you can see his normal neck under the fake slit-throat neck, and once or twice, you can even see a little device that was meant to shoot out smoke from a cigarette in the fake appliance.
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: In the final fight scene we see Matt hitting a guy in the arm with a piece of wood. If you watch closely when he hits him the wood vibrates, indicating it is made of rubber.
Revealing mistake: When Chris and Jamie are on their day date in the diner, Jamie has a grilled cheese sandwich with ranch on the side. However it is obvious that the bowl of ranch is empty. (00:33:10)
Revealing mistake: When Jake "kills" Chelsea, and his phone starts to ring, the phone is blinking, and vibrating. It falls to the floor, and Jake reaches for it, when he does this you can see that the phone is not actually ringing, because the screen is dark and the screensaver is active.
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.