Revealing mistake: Towards the end of the movie where Veronica Roberts hands are tied behind her back, she goes over to the mulching machine to cut the rope free. But as she hits the blades they bend and move like rubber.
Revealing mistake: While Nick and his father are driving home with the giant baby behind (an obvious green screen projection), the baby breaks a tube from the car and throws it to the floor. Nick bends down to pick up a prop located offscreen, to add up for realism, but the problem is that he bends down before the baby throws it, so the whole trick is revealed.
Revealing mistake: As Ripley and Dillon lure the Alien into the piston chamber, Morse closes them just in time. Watch closely - he misses the switch. (01:38:15)
Revealing mistake: When Nick returns to his apartment (after being made invisible) he immediately closes the curtains. Although he is invisible, we should see the curtains pulled closed at the point that he grabs them with his hands. Instead we see them close from the top, revealing they were closed either by a crewman or an automated process. Note: The curtains are NOT meant to be automatic in the film as we see and hear him physically close them.
Revealing mistake: The opening text describing how London is badly flooded because of heavy rains and global warming is accompanied by an aerial shot of the city. From the level of the Thames it is clear that this wasn't even filmed at high tide, let alone during a flood. (00:00:45)
Revealing mistake: When Daniel belly-lands his plane the rotors get totally bent. When the plane bounces back the rotors happily rotate again, revealing their rubbery consistency. (00:03:15)
Revealing mistake: Toward the end of the film, there's a scene were Dick hits a Spango soldier and Dick shakes his hand in pain. If you look over his right shoulder, you see two people fighting but there's no physical contact being made.