Corrected entry: When the T-1000's truck crashes in the aqueduct, it explodes, and the T-800 gets his gun ready. We see the flaming tire rolling out, the T-800 aims at it, thinking it's the T-1000, but a few seconds later realizes that it's just a tire, and puts the gun away. He has remarkable optics and should be able to see that a tire is in no way the enemy.
Knever
15th Sep 2014
Correction: The T-800 knows the T-1000 can mimic anything it has touched. Until he is sure the T-1000 has not mimicked a rolling tyre to get past his defences, it is a legitimate target.