Continuity mistake: When Tim and Lex first arrive, Lex is wearing a purple tank top with a colorful design, and in the following scenes or even between consecutive shots her tank top changes to completely different designs, though it's the same style of tank top.
Continuity mistake: When Simon Phoenix gets out of the sewer system (after gunning it out with the rebels and John Spartan) his left eye has the brown contact and his right eye has the blue one after going the entire movie with those contacts in the opposite eyes. (01:30:25)
Continuity mistake: When the two Ninja robots fight, the robot on the left loses his head before the blade strikes.
Continuity mistake: In the scene where Jessica is in the shower crying and the power goes out, she gets out, puts on a white t-shirt and goes walking around the house. When she walks through the garage door and grabs a hammer, there is blood on the front of the t-shirt (right at or a little lower than her waist and looks like someone grabbed the shirt with a bloody hand). Where did it come from? She didn't run into anything or anyone. [There's a deleted scene showing her step on a piece of glass. She gets blood on her hands when she pulls the glass out, then presumably wipes her hands on her shirt. Still a mistake, as removing that scene means the blood appears from nowhere, but there's the reason.]
Continuity mistake: In one scene, Mark Hamill jumps into the passenger side of a car while running from men with guns; the men shoot at him, and the passenger side window is shattered by the gunfire. A moment later, once the car has gotten away, the passenger side window is intact.
Continuity mistake: When Mike Rogers's daughter is sleeping on his lap and his wife comes to ask him if he's coming to bed, in one shot the daughter is way up sleeping on his shoulder and in another, back down on his lap.
Continuity mistake: In the scene where Beldar has finished giving Gladys a driving lesson and they are sitting in the car, you can see that he has nothing on his lips. After Gladys kisses Beldar, you can see red lipstick. As the shots go from Beldar to Gladys and back, you can see the lipstick disappear and re-appear.
Continuity mistake: In Devo 4 Luigi's button unbuttons and rebuttons itself. (01:18:05)
Continuity mistake: After Bruce proposes to Andrea and he drops her off at home, she is wearing the ring on her left hand when she is inside his car. But once she steps out of it, the ring completely disappears.
Continuity mistake: At the Riverman's place, Julie grabs a piece of glass from his foot and stabs his left hand with it, later when she is finishing his transformation and stabs a knife in the same hand, there's no sign of the earlier injury. (01:02:50 - 01:04:45)
Continuity mistake: In the scene where Gard telepathically fights an alien, you see his nose bleeding. Then the alien explodes, and in the next shot of Gard's face, the blood under his nose has vanished, only to return in the next shot of his face.
Continuity mistake: The two buddies are eating and listening to the exposition news item about the "Zone of Personal Freedom." In the last part of the scene ("Who do you suppose are gonna call to clean that up?" the straw inside Chad McQueen's milkshake keeps swapping position between the middle and the rim of the glass. (00:03:20)