Continuity mistake: When John and Catherine are in the hangar at the runway, the Cessna's tail number is N3035C. When the plane is shown in the air, the number is N3973F. When they land, the tail number has changed back to N3035C. (01:22:25 - 01:25:50)
Character mistake: When the Terminator scans the doorman at the bar, a display comes up categorizing each piece of the doorman's clothing. The word "briefs" is misspelled "breifs". (00:13:40)
Continuity mistake: In the cemetery scene when the Terminator uses a bazooka to fire a missile at the TX, the shot that shows the TX being launched up in the air and hitting a gravestone should have shown the car used by the TX and the road where the car was parked. In the next shot, when the TX gets up and starts to run after the hearse, the road reappears but not the parked car. (01:04:40)
Continuity mistake: In the scene just after Kate Brewster's dad has been shot he says to go to his office. On the way there you can see his jacket is undone and you can see his shirt and tie. Yet when they enter the office it shows him sitting down with his jacket neatly buttoned up. For a guy who just got shot and is running from machines, I doubt he would worry about buttoning up his jacket. (01:16:40)
Continuity mistake: As the hearse comes out from under the truck while they are trying to get rid of the Terminatrix, you can see that the windscreen is intact. But as they drive down the hill it's missing. (00:59:10)
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)
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)
Continuity mistake: During the scene when Arnold carries the casket, a shot shows many bullet holes on its front side. Two shots later, these holes have disappeared. (00:54:45)
Trivia: When the Terminator reboots himself in the hangar in order to get rid of the influence of the T-X, the scrolling text includes "Quicktime Player - Access" and "MP3.com - No Access." (01:30:15)
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 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)
Other mistake: When Kate is viewing the T-X on the security cameras, the two views of the corridor do not match each other. For example, the first camera shows glass walls, whereas the second shows wall panels. (00:23:30)
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)
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)
Continuity mistake: When John is threatening to kill himself, his finger alternates from being on the trigger of the handgun to being on the trigger guard from shot to shot. (01:04:55)
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: 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: 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 John and the vet arrive at the mountain in their Cessna, the sun casts a shadow straight under the plane. The very moment they walk through the door into the hangar integrated in this mountain (which in the film is immediately after they landed), their shadows are in front of them indicating a low sun. (01:30:45)
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)
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