Continuity mistake: The Terminator is following the crane at the rear corner when the TX moves the arm out allowing the hook to hit parked cars and telephone poles. During the overhead shot there is no Terminator behind the crane.
Continuity mistake: During the chase scene with Kate's truck and the T-X in the crane, the driver's door disappears and reappears several times.
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)
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: During the chase scene when the T-X is driving the crane, the legs of the crane are out in some shots and retracted in other shots.
Factual error: During the scene where the Terminatrix remotely controls the police and rescue vehicles, we see the police car's shifter drop one notch (from park to reverse) but the car takes off going forward, throwing a cop out. (00:28:55)
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)
Continuity mistake: When John talks to Catherine while sitting at the table in a moving RV, watch the explosives he is working on. Position of the charges changes with every shot facing Catherine. At one point they completely disappear from the table leaving only the Glock 18 handgun, and just a second later the table shown full of explosives again. (01:09:55)
Continuity mistake: When the Terminator breaks into the van the broken glass changes from smashing it to getting in.
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.
Other mistake: After the T-X is hit by a truck, her primary weapon is damaged and she is seeking a replacement. As she is cycling through available weapons, dummy text is below the weapon name. The text reads: "TEXT regarding the current weapon is being displayed in XP040 weapon sub-window. Important information regarding this weapon is being displayed here." (00:59:52)
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.
Audio problem: When Arnold makes the slow turn at the cemetery from Dr. Silberman's view, he starts firing his weapon at the police but we don't hear it. In the very next shot, he repeats the same action but we hear it this time.
Audio problem: When the T-X steals the car in the beginning, she's driving with both hands on the steering wheel. The car appears to be a 2001-2003 Lexus SC which has a 5-speed automatic transmission (without steering-mounted paddle shifters). However, the sound from the transmission indicates it's a manual transmission rather than an automatic transmission. Therefore, she cannot be shifting gears while having both hands on the steering wheel. (00:07:15)
Continuity mistake: Just before both Terminators are thrown in the bathroom, the T-850 grabs the T-X by the shoulders, but in the next shot it's by the arm.
Other mistake: When General Brewster tells John and Kate to go to Crystal Peak, Arnie confirms its location as "52 miles north-east, bearing zero-point-five°." Later, when flying the plane, Kate says aloud "all right, oh-one-five°, 52 miles." That's a 10-degree error, for 52 miles, which would put them somewhere in the ballpark of 20-25 miles off-course. (01:17:25 - 01:29:40)
Continuity mistake: After Kate's dad gets shot at the base the blood and the wounds and the holes in his jacket disappear.
Continuity mistake: When John and Kate are in the Crystal peak bunker, T-X smashes through with a helicopter, and there is an external shot of the chopper's rear blade spinning, but when T-X exits, the rear blade is gone, then in the wide shot of T-X it reappears. (01:32:35)
Visible crew/equipment: In the shot just before the ambulance truck runs over Arnold's police motorcycle, you can see on the right-hand side of the screen a film/crew member truck that doesn't belong in the shot. (00:34:40)
Other mistake: At the cemetery, Arnold shoots up the police cars. We then see his point of view, showing no human casualties. The problem is, he is standing up and further back from the drive. The shot from his point of view is down low and near a police car.
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