Revealing mistake: When Will Smith throws a rock at the escaping Edgar alien, if you watch the rock after it hits the alien, it disappears after hitting the ground. (01:23:20)
Revealing mistake: When Sarah is making her way to the baby Stego, just before she gets on her belly and crawls closer to it, you can see the kneepads she has on.
Revealing mistake: Right after they "create" Leeloo and she is lying in the plastic cage with the little straps on her, the guy approaches her and says, "if you want out, you are going to have to work on your communication skills." She gives him a dirty look, and then you can see a huge hole, pre-cut in the glass, that she busts her hand through. (00:29:47)
Revealing mistake: When they are learning how to throw knives, the guy gets one thrown through his hand. If you watch this carefully, blood starts coming out of his hand before the knife hits. (00:28:45)
Revealing mistake: When Hillard throws the basketball at Ripley, you can clearly see the white tape on Ripley's hand which was used to catch the ball. (00:23:10)
Revealing mistake: When they get Renn back into the room after he has been shot in the face with something acid-like, the smoke comes from behind his head. Not his face.
Revealing mistake: In the big speedboat scene near the end, when Cage is hanging off the side of the boat the stunt double is obvious in wide shots, where the entire boat is visible from above. He has big long sideburns, which are in stark contrast to Cage's short hair, visible in closer shots. At various other points in the movie, stunt doubles appear of much different physical builds than the two actors, with markedly different hair colours, and I seem to recall that at least one of them had facial hair.
Revealing mistake: When Ellie is traveling through space, she unbuckles herself from the chair to retrieve the compass. The chair then breaks off from the violent movements of the space pod, and slams up against the ceiling. At this moment, Ellie is in deep space in a completely weightless environment. She is floating, suspended in the pod and the chair itself floats a bit before being slammed upwards by the force of the pod movement. But, some small parts of the chair (bolts, etc.) are seen falling directly down, towards us in obvious full gravity; revealing that the scene was actually shot in normal gravity.
Revealing mistake: Although the revelers in Dallas are celebrating the 4th of July, it's very obvious by the characters' clothing that the film was shot during cold weather.
Revealing mistake: When the Cyborg Master enters the police station, he shoots two police women before a third cop starts shooting back with a shotgun. The third shot he fires hits the wall behind the cyborg, and the entire wall wobbles as the squib goes off to make the hole in the wall.
Revealing mistake: When Jude Law needs to pull himself up the flight of stairs to reach the intercom, you can see him use his hips, despite the fact that he is paralyzed from the waist down.
Revealing mistake: In the scene where Sam Neill has set the bomb to blow up the rescue ship, Laurence Fishburne comes running up to the sealed door of the Event Horizon whereupon it shakes back and forth like a stage door would.
Revealing mistake: When Fred Randall is in the motion sickness machine (the one that spins around horizontally), you can see the three people in the observation centre above at one point. Look at them closely, because they are fake.
Revealing mistake: Near the end of the film, when Dacascos is fighting that other guy from Hong Kong, if you watch just after he does this smart kick, the guy's actually an Asian stunt double, and a lot shorter than Dacascos (best seen on the DVD putting it in slow motion just before).