Continuity mistake: The T-1000 punches his body through the window of a helicopter to get inside. An instant later, the hole in the windshield is gone. (01:56:35)
Terminator 2: Judgment Day (1991)
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Directed by: James Cameron
Starring: Arnold Schwarzenegger, Robert Patrick, Xander Berkeley, Edward Furlong, Linda Hamilton
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More mistakes in Terminator 2: Judgment Day
Dr. Silberman: You broke my arm!
Sarah Connor: There's 215 bones in the human body. That's one.
Trivia: The minigun that the Terminator (Arnie) uses in the Cyberdyne scene was so heavy that Arnold Schwarzenegger was the only person on set strong enough to lift it.
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Suggested correction: In one of the special features on the Blu-ray version, director James Cameron can be seen holding and shooting the minigun, while saying: "Arnold's gonna love this."
Question: Arnold said that when the T-1000 took the form of someone he would likely terminate the person being copied. Why didn't the T-1000 kill Sarah? He had been in physical contact and could mimic her voice. There was no reason to get her to call John at the end, he could have killed her immediately.
Answer: In addition, another deleted scene shows that the T-1000 was damaged by being frozen and thawed and was having difficulty maintaining form. He wanted to be sure he wouldn't mess it up.
What's especially odd about the scene is that both examples we gave to help answer this question were actually deleted from the original release. When I first saw this movie in the theater when it was released I had the exact same question, it wasn't until I saw the Special Edition with the deleted scenes reinserted that it made sense.
Answer: It comes down to the difficulty in impersonating someone John actually knows. The T-1000 had failed impersonating someone close to John when he got the name of the dog incorrect. A deleted scene (re-incorporated into the film in the Director's Cut) shows him checking the dog's tag afterwards, so he is aware exactly how he was outsmarted. Keeping Sarah alive is a way to avoid this sort of mistake. She is more useful to him at this point alive. He is not expecting her to endure his torture and he certainly isn't expecting her to be able to fight him afterwards.