Revealing mistake: When the T-800 is being attacked by the SWAT team in the Cyberdyne building, notice that in most of the shots (i.e. when he is being shot in the face, and also when the camera is behind him) Arnie has been replaced by a dummy (his skin is unnaturally pale). When the T-800 pulls out his gun and starts shooting the SWAT team, it's now back to the real Arnie, and notice how none of their bullets are now hitting his face and body, even though shots are still being fired. (02:00:20)
Revealing mistake: When the T-800 and T-1000 first begin to fight in the steel mill, the T-800 puts his fist through the T-1000's head causing him to revert to liquid metal form, then as the T-1000 is throwing the T-800 from side to side you can obviously tell that the T-1000 is just an actor wearing a highly metallic mylar suit in some shots. (02:14:10)
Revealing mistake: Early on in the film, the T-800 sees John riding down the aqueduct. He stops, and a blue car skids and is hit by another on its right side near the rear wheel. As the T-800 sets off in pursuit of the T-1000 and John, a blue car skids and is hit on its left side. Guess what? Its the same car as earlier. You can tell by the massive dent above the rear wheel. (00:29:20 - 00:34:40)
Revealing mistake: When the tanker truck (liquid nitrogen) is sliding along the ground the sparks can be seen not to be coming from directly under the tanker but ejecting from small canisters slightly higher up the vehicle. (02:02:55)
Revealing mistake: In the scene where Sarah opens the skin and endo skull of the Terminator, you have a shot where you see the reflection of the Terminator, Sarah and John in the mirror. When the camera takes a shot from the back of the Terminator, you see the back of a puppet on which the real Sarah is performing the operation. In the reflection you see no camera, the real Terminator, and doubles of Sarah and John. Especially the moment Sarah puts down the screw driver, the reflecting screw driver is put down a fraction later than Sarah's. (01:07:09)
Revealing mistake: Special Edition. The biggest giveaway about the false mirror in the 'chip removal' scene is that John's 'reflection' has his back to the mirror when Sarah is drilling, whereas the real John is standing side-on to it. Also notice that John nervously looks at the camera in this shot. (01:11:50)
Revealing mistake: In the scene when the T-800 first comes through the time machine, in the shot where he stands up. If you look towards the right side of the bowl shape that's melted into the ground, you can see that the bowl shape is actually above ground. There's a shadow being cast by it on the right side. (00:06:30)
Revealing mistake: After the Terminator has jumped on to the truck and shot the T-1000 full of bullets he pulls the wheel to the right so that the tanker truck skids and falls but if you look closely you can see that the stunt double for the T-101 merely fakes he's pulling the wheel, and then holds onto the cab and you can see the stuntman for the T-1000 inside the cab turning the wheel before it flicks to the wider shot. (01:48:10)
Revealing mistake: When the T-800 is reloading his shotgun in the alley with John, he reaches into his pocket, presumably to grab shells for the shotgun. In the shot with him and John in frame, his hand comes out of his pocket empty, but we hear him load another shell into the gun in the next shot.
Revealing mistake: When the cop by the vending machine is about to be killed by the T-1000, the back shot of him reveals he's been replaced by an obvious mannequin. His perfectly trimmed hair along the nape is now long and messy, and a long stitch appears along his sideburn and down to the neck.