Revealing mistake: There is a shot from the Terminator's POV, before he rams the blue car with the semi truck, in which you can see that the car is already damaged. No slow-mo is required to see this, but it is a blink-and-you'll-miss-it shot.
Revealing mistake: When the Terminator's flesh is burning off in the burning tanker wreckage, you can see the wire moving the Terminator puppet around. (01:29:45)
Revealing mistake: In the second future war sequence. At the end of the HK/car chase, the gunner in the car has been replaced by a very obvious dummy. (00:19:55)
Revealing mistake: When Arnie drives through the police station, the cop is just sitting there. Not only that, his skin is a lot paler. It is too easy to see that it is a dummy. (01:00:00)
Revealing mistake: During the final chase scene, Sarah loses control of the pickup and it rolls over, as it rolls you can plainly see that it has no engine. (01:26:05)
Revealing mistake: When Arnie is being pulled under the semi-truck, his foot is caught and the green-colored dummy leg is revealed. It can't be a special effect sock since green screening was not in practice in the early 80s. (01:23:05)
Revealing mistake: At the end of the film, at the split second we see the ambulance workers zip up Reese in the bodybag, his right eyelid flinches a bit. (01:39:45)
Revealing mistake: The second future war segment: During the wide shot where Kyle and the female soldier are following the HK Tank, most of the plasma shots fired from said tank seem to come from above the gun turrets and not out of the barrels. (00:18:45)
Revealing mistake: In the second 'future war' sequence where a female soldier is practically vaporised by a passing HK tank, watch her flaming remains. You can see the (exceptionally fake looking) head of the stand-in dummy tumble away from the body. (00:14:15)
Revealing mistake: After the Terminator (in his police car) discovers Reese and Sarah in the car park, Reese hot-wires the car and starts to speed away. After that, there are several shots of Reese driving the car. It is easy to see that it's not Michael Biehn, but a stunt double who doesn't resemble Biehn at all, and who is seen in several other shots during the movie. (00:48:10)
Revealing mistake: When the Terminator invades the police precinct to kill Sarah, there is a part where he shoots a cop in the butt with his shotgun (right before he short circuits the building's wires). If you pay attention, the cop is smiling and giggling as he takes cover - before he is blasted. (01:00:45)
Revealing mistake: In the scene were Kyle and Sarah are speeding away from Arnold in the truck, Arnold starts shooting the truck with Kyle driving. When Sarah and Kyle decide to switch (passenger, driver), the camera shows them switching but there is a stuntman in place of Kyle. If not he has gained some weight in the cheeks and grown a big mustache during the course of 2 seconds. (01:24:30)
Revealing mistake: When the Terminator takes out his "fake" human eye, he looks into the mirror. It is very obvious it's a dummy: 1) He has unnatural movements. 2) His skin is a lot paler, and not shiny. 3) His hair style has changed. It's easier to tell in the following shot, as he looks closer to the mirror that he's changed back to "real." (00:59:55)
Revealing mistake: In the shot of the Terminator skidding down the street after being rammed by the pick-up, it is a dummy. It's even missing a hand.
Revealing mistake: In the scene where the tanker explodes, we see the cabin of the truck getting blown up. Then in the next shot, we see no flames have even reached the cabin, and it explodes again, this time with flames.
Revealing mistake: After Reese blasts the Terminator with his shotgun while at the Tech-Noir nightclub there should be huge holes in the Terminator's shirt and jacket, but there are none.