Continuity mistake: When the T-X steals the car in the beginning, she's weaving in and out of traffic and from her view we see that she is coming up to a truck in the right lane. The view changes to view from outside the car and she's turning into the right lane and driving straight ahead. The truck is gone. (00:07:50)
Continuity mistake: When the TX runs a red light in the stolen Lexus, a cop stopped at the red light does a one-eighty to begin pursuit. When the TX pulls over, the license plate on the cop's squad car is not the same as when it was stopped at the red light. (00:08:10)
Continuity mistake: When the TX pulls up to Jim's Burgers, notice the umbrellas in front of the store and the "in-out" arrows on the road. Asking for Jose Barrera at the drive-through speaker, she drives in and shoots him from the passenger side of the car and exits on the same side of the building she came in. You can see the front of the store with umbrellas upon entry and exit, yet the speaker has disappeared. (00:16:30)
Continuity mistake: When Kate gets the phone call in the middle of the night to goto the pet clinic, her phone is a white+blue Nokia 8310. Later on when she is calling from the back of the pet truck and it loses reception and the camera goes onto the phones, its now an all blue Nokia 51xx (I think) series phone. (00:17:30 - 00:29:25)
Continuity mistake: After Kate throws John in the dog cage, moments later she returns to say, Mike Kripke's Basement. The first two cutaways to John there are no bars in front of him. On the third cutaway, there are bars. (00:21:05)
Continuity mistake: When the T-800 first drives the truck into the Vet clinic, the truck has crashed into the building, so that the doors are inside the clinic. Yet in the very next shot when he gets out, the truck has moved back so that the doors can open cleanly, without the T-800 backing the truck up. Then when John escapes the cage and he runs into the T-800, you can see the truck inside the clinic. It is now more than halfway inside the building. (00:24:15)
Continuity mistake: When the Terminator puts the tyre wrench in the doors of the van he bends the wrench. But later in the desert it's bent differently. (00:25:15 - 00:41:40)
Continuity mistake: When Katherine Brewster is initially saved by the Terminator from the TX and thrown in the back of the truck, her knees are wet, then drier and drier but then wet again showing the sequence we see was shot at different times. (00:25:30)
Continuity mistake: When John crashes the Toyota truck into the rear of the silver Taurus, the car's bumper cover lands on the ground to the right of the car, between it and a parked car. When John drives away from the accident, he drives between the Taurus and the parked car, but the cover isn't there to be run over- it's now tucked under the Taurus at the right/rear corner. Then, as the crane smashes the Taurus, the cover is now well to the car's left, in the lane of opposing traffic. (00:31:10)
Continuity mistake: After John crashes the truck into the silver car, the empty emergency vehicles approach from behind. At the right, a red sedan is shown from the rear with a broken tail light and mangled bumper. A few shots later, the crane comes around a corner and hits this red car, but before the actual collision, the tail light and bumper of the car are fine. (00:31:55)
Continuity mistake: When the remote cars are crashing into John Connor's truck the up close camera angles do not show the same damage on the front and sides of the police cars as the far away angles do. (00:32:40)
Continuity mistake: When Arnold shoots the crane tires, the motorbike is shown to have a windshield. Right after when the T-X knocks him off the bike the windshield is missing. The next shot is the ambulance hitting the bike and the windshield is back. (00:33:15)
Continuity mistake: During the crane chase the amount of damage to the van changes drastically throughout the chase. (00:33:35 - 00:35:00)
Continuity mistake: The crane's arm is draped in cables, wires, and even a partial telephone pole right before Arnold is dragged through the buildings glass facade. The next shot before impact the crane's arm is clean. (00:34:00)
Continuity mistake: The driver side rear view mirror is broken on the police car as it chases the tundra truck onto a front yard, but after leaving the yard the mirror is back intact. (00:34:55)
Continuity mistake: After the first two fireman jump out of the fire truck, Arnold climbs in and says "Get out" to the other fire fighters. When the next two fireman jump out, the first two are not seen on the road behind them. Also, when Arnold changes gears on the fire truck none of the four fire fighters are seen on the road. (00:36:10)
Continuity mistake: After Arnold has taken the fire truck, John is chased by a single police car. The overhead shot pans left to show the crane approaching from John's right. This shot shows the crane arm is pointing rearward. But when the crane bursts through the wall a split-second later, the arm points forward. (00:36:20)
Continuity mistake: During the big chase scene (a little after the Terminator collided with the fire truck), the crane crashes through another glass store front. When it emerges and turns, the hook is tied to the front of the crane, just as it was when the T-X first got it. (00:36:30)
Continuity mistake: When the Terminator jumps onto the roof of the truck that John is driving, the damage he causes changes when we see the truck driving off into the distance. (00:37:30)
Continuity mistake: When the Terminator gets in the Toyota Tundra with John (after destroying the crane that the T-X was in), you can see that there is no seat belt in the seat belt holder. But when the Terminator is examining John for "brain trauma", there is now a seat belt in the seat belt holder. (00:38:00)
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