Revealing mistake: During the opening credits, the Terminatrix steals a woman's car. She uses her mobile phone to dial a number and successfully connect to a computer. However, you can see that the phone has no reception, making a phone call impossible. Battery life is the right hand gauge, and signal strength is the left hand one, with no bars. (00:07:25)
Revealing mistake: When the T-850 drives the truck into the building, from the side view the truck misses the silver car completely. Yet in the next shot it appears to hit it, and the car lurches (somewhat unrealistically as if pulled by a cable) to the left and explodes. (00:24:00)
Revealing mistake: When the T-X is trying to get Kate out of the car, she rips the door off the truck and pulls Kate out. Kate goes flying through the air and lands on the tarmac. However, if you watch the floor closely just as she lands, you can see that the floor actually wobbles and shakes. It is obviously not real. (00:24:35)
Revealing mistake: During the crane chase scene, the T-X is shown changing gear twice. Both times, the arm is definitely not Kristanna Loken's. It is much thicker and has lots of hair. In another shot, during this chase her arm is shown and it is different. (00:33:50 - 00:34:35)
Revealing mistake: When they are driving the tundra after the big champion crane crashes, the Terminator gets in the truck. If you look closely, the truck's shifter is all the way in park. (00:37:55)
Revealing mistake: Look carefully at the metal bar with which Arnold secures the doors of the van with the Vet inside. As he bends the bar straight so he can open the doors, it vibrates a little indicating it is a rubber fake. (00:43:15)
Revealing mistake: When the animal rescue truck crashes through the seminary gate, the sparks that fly come a few seconds after the truck hits. (00:48:25)
Revealing mistake: After the Female Cyborg kill's the two agents in the car, there is a crane shot of her car turning around in the middle of the road where you can see see the mark in the middle of the road showing the driver where exactly to turn. (00:53:45)
Revealing mistake: During the entire hearse chase scene, the transmission shift lever is in park. (00:55:00)
Revealing mistake: When the Terminator uses the the G-36 assault rifle, the muzzle of the rifle is fitted with a maneuver ammunition device which shows that he uses no real ammunition, but a variety that just makes "bang" sounds without a bullet. (00:55:35)
Revealing mistake: When the hearse has no roof it is obvious at one stage that it is not Arnold sitting behind the steering wheel but his stunt double. (00:57:10)
Revealing mistake: When John, Kate and the Terminator are in the Winnebago, there is a shot that's facing Kate where John says to her that they're going to pull the plug on Skynet and that the bombs won't fall. In this shot, look at John's hair. The green screen is reflected onto it. It is visible throughout the rest of the scene. (01:09:45)
Revealing mistake: When Arnie body slams the TX into the toilet, the fact that it's a dummy is blatantly obvious, even at full speed. (01:20:15)
Revealing mistake: At the end of the movie when the missile hits and explodes on the city in the background we see the buildings closer to the screen (the barn and the small office building) get demolished by the shockwave, but all the trees surrounding these buildings don't even move. (01:41:15)
Revealing mistake: During the big fight between the two terminators, the male grabs the female and smashes her head into a toilet. If you watch carefully as soon as her head hits the toilet it cuts seems to cut to another shot. (Easier to see in slow-motion).
Revealing mistake: In the driving scene after the first run in with the TX, the T-850 is driving the truck, John is in the passenger seat, and Kate is locked in the back. The T-850 has to cut open his abdominal area to pull out the nuclear power cell that has been damaged, and dispose of it. In the shot before he begins cutting, you see the T-850's chest is flat as it should be under his shirt. But once you see him begin cutting into his abdominal area, the top of his chest has a ridge. This is an obvious show of the prosthetic chest the actor scoot up against to be as his chest and house the fake skin, metal, and power cell for the scene.
Revealing mistake: When the Terminator, Kate, and John are escaping the cemetery, they come down a winding road and drive in front of the camera in a hearse. Both Kate and the Terminator have been replaced by stunt drivers.
Revealing mistake: When Arnold is breaking a glass cover to a wall display case in the military base, the glass breaks just before his hand touches it.
Revealing mistake: During the big fight, the female robot turns her body around to face the male terminator. When she turns around, watch her legs as you can see there is nothing in them. They are just pants with no legs in them.
Answer: He was originally in talks - and by some reports, did appear on set for some period of time. While factual answers are in short supply (for obvious reasons), there appears to be a connection to his drug-use habits. For instance, he was checked into a hospital (LA Cedars Sinai) for what friends feared was a drug overdose in April 2001, and has been in and out of rehab over the years. Chances are the production company was unable or unwilling to get completion insurance.
Rooster of Doom