Revealing mistake: Doug has the hologram when inside the alien nuclear reactor, and a bunch of bad guys encircle the holographic image and fire at it at point blank with automatic weapons, and not one bad guy gets hit. If they encircled the 3D image, at least a few bad guys would have been hit by their own men. (01:37:00)
Revealing mistake: When Arnie uses the dead body as a human shield, watch the corpse's mouth - he grimaces every time he gets hit (the squibs hurt). (00:31:00)
Plot hole: When Arnie is taking off his woman's head he presses a switch by the ear and the ear comes out. The device reaches right inside the fake head and there would be no room for Arnie's real head. (00:46:00)
Revealing mistake: Pay attention to the drywall on Quaid's left just as he exits the bathroom to a rain of gunfire. If you look closely there are discolorations in the paint where explosive charges are packed and set to detonate in lockstep with the shots - creating "bullet holes". (00:25:00)
Other mistake: In the scene after Arnold kills the construction hoods he goes home to Sharon Stone and tries to convince her that it was real by showing her his bloody hands with which he was grabbing her shoulders, yet there is no blood on her or the appliances he turned off upon entering the apartment.
Continuity mistake: When Arnold Schwarzenegger and Rachel Ticotin are being attacked by the giant drilling machine, Arnie grabs a pneumatic drill and cuts the main oil line of the machine, getting the left side of his face and his shirt doused in oil. Yet throughout the remainder of the film, there is no oil on Arnie's face or shirt. (01:39:55)
Continuity mistake: When Arnie is giving his info about what his Mars woman should be like, there are two things to look at on the screen in front of him: The screen zooms in on the woman's face, cuts back to Arnie, cuts back to the screen when it zooms in on the face again. Also, look at the sequence when he's selecting her personality: "demure" is lit up before he mentions it. (00:17:35)
Continuity mistake: When Arnie uses the hologram projector in the alien reactor, there is a brief shot of him facing his own hologram image. Note that the gun and the projector switch positions in Arnie's hands - the bloodstain on his shirt's sleeve does not. (01:37:55)
Continuity mistake: In the escalator scene when Arnie uses a corpse as a shield, there's a close-up of Arnie's gun firing, and we can see there's blood on the side of the corpse's head. However, in the wider shots there isn't. (00:31:00)
Revealing mistake: When Quaid throws the bullet-ridden body down the escalator, you can see Richter and Helm have both been replaced by stuntmen. Their hairstyles are totally different. You can also see that the 'dead' man is still alive as he's flying through the air (slow motion helps). (00:29:55)
Continuity mistake: Soon after the woman costume malfunctions, you can see the blue screen through the windows, which in the next shot is coloured orange.
Continuity mistake: When Arnie meets Bennie, he takes him to his cab and opens the door for him to get in. At that time, there's an explosion, and Bennie pulls the door down to shield himself and Arnie. The camera switches to behind them, and Bennie is just now pulling the door down. (00:52:50)
Revealing mistake: When Arnold jumps through the glass, you can definitely see a change in his face and body shape. (From the stuntman). (00:30:05)
Continuity mistake: When Quaid finishes Gary and his fellow agents, the concrete wall behind him shows a bloodstain. The only opportunity for it to appear there would have been as Quaid hits the moustache man in the face; but this guy stands up again, and it doesn't look like he cracked his skull (which would have caused a stain). (00:22:30)