Revealing mistake: When the group is being attacked in dining hall B, they are trying to get the doors open. J.D. is firing at two zombies and turns away to open the doors himself because Kaplan keeps forgetting the code. If you look closely, one of the zombies reacts a second late from being shot down.
Revealing mistake: On the DVD commentary, the director says that he only had a handful of the experiment cubes in Dining Hall B and used cardboard cutouts for all the others. This is most obvious when Rain is walking through the hall alone to check on the first zombie: the cubes in the distance don't match the rigid rows of the ones in the foreground.
Revealing mistake: In the scene where Milla Jovovich shoots the dogs with the 9mm Beretta she obtained from the dead security guard, a closeup of the gun is shown. Watch the cycling of the slide and you will notice that the ends of the bullets are crimped, indicating the use of blanks.
Revealing mistake: Near the end right before the end credits we see the streets of Raccoon City completely destroyed, but if you look carefully you can see the background is repeating itself street by street. The best way to notice this is to look at the firetruck right before the end. There is a firetruck, a news van and a bus. Now look to the next street on the other side of the road and the exact same cars appear there.
Revealing mistake: The Red Queen tries to goad Alice into killing Rain in the laboratory, with the Licker pounding against the glass outside. Just after they escape the room, the Licker breaks in. Watch its jump very carefully. you can see the exact invisible "line" where the Licker stops being a guy in a suit and becomes computer generated. It even changes colour.
Revealing mistake: As the team gets off the subway and heads for the door into the Hive, the Red Queen's POV shows a nifty infrared display in the lower left corner that marks their locations - it obviously matches up to the ones on screen. But the dots are static, while the people are moving.