Revealing mistake: Before Paxton is hit with the 3 prong instrument, you can see the bloody holes already on his shirt. (01:01:20)
Revealing mistake: When the team descends to the bottom of a hole and one female member is first wounded and then dies. After checking her pulse, the others pronounce her dead and begin moving out. As the last one is leaving, the "dead" actress lifts her head up off the ground and looks left. (01:13:10)
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: Towards the ending in the strip club, Locus is getting arrested. When it shows his hands being cuffed, it's not his hands but those of Charles in an earlier scene. You can tell because the person is wearing long red sleeves and a Rolex.
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 Shane is fighting the two ninjas, they are armed with expandable batons, which are tubular hardened steel. During the fight sequences you can see the plastic batons bending repeatedly.
Revealing mistake: In the scene where Jane is trying to run over John with her car, she jumps from the driver's seat while John is in the back of the car. Right before the car crashes through the fence, you see John through the back window. You also see a head in the driver's seat, even though no one is supposed to be driving the car.
Revealing mistake: When Lisa stabs Jackson in the throat with the pen, if you watch closely, you can see that she is still holding the pen when she pulls her arm back, but in the entire rest of the scene the pen is still stuck in his throat. If you watch it in slow motion, the mistake becomes even more obvious. (00:56:15)
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: The "metal" axe in his hand bends as Jacob flies through the window of the tower and smashes into 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 Kadaj kneels in front of Rufus Shinra, watch his sword. As he kneels, it goes through the floor.
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: 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: 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: 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)