Revealing mistake: This is toward the end of the movie. The scene where Dr. Giggles has a waiting room full of his victims. If you watch closely, you will see the blonde corpse blink her eyes.
Revealing mistake: In the scene where Tom Cruise is yelling at/questioning Dawson at the jail in the witness room, immediately after Cruise gets up in Dawson's face and says, "I'm the only friend you've got," Cruise is supposed to simply turn and walk away. But you can see from the angle behind Dawson that Cruise' first step is a step down, indicating that he was on a pedestal of some sort to bring their heights closer together.
Revealing mistake: When Lance Henrickson's character gets killed, there's a close-up of his flashlight lying in the snow with a rivulet of blood flowing past it. It's very obviously fake snow, since there are snowflakes floating on top of the blood.
Revealing mistake: Michael Douglas almost has a head-on collision with a bus as he chases Sharon Stone down the highway. If you look closely as he slams on his brakes, there are many skid marks in that exact same spot in the road from previous takes. (00:41:05)
Revealing mistake: In the scene when Crow Horse and Levoi are running from Coutelle and the "GOONS", they go off-road toward the stronghold. You can see in the grass the tire tracks from all the other takes. Especially as there is no reason any vehicle would leave the road at that place otherwise.