During the Helicopter chase you can see several shots where the T-1000 seemingly has extra arms, and one scene in particular is right before the crash you can see the T-1000's left arm shooting the MP5k and another left arm flying the helicopter. This isn't signs of the T-1000 "growing" a new arm though. If you stop at around 01:58:50 you see the third arm and if you look you can see where it attaches to the shoulder right before it become the police officer, The "ghost arm" is wearing a light jumpsuit instead of the dark police uniform.This looks like a layering issue between two shots.