When the T-1000 is chasing Arnold and John in the flood canal, he hits a concrete wall and the truck explodes. As Arnold and John are driving away from the explosion, it is quite clear that its not Arnold Schwarzenegger and Edward Furlong on the bike. Its a stunt double for each of them. OOOPS!
That's not a slip-up. You can tell that they use stunt doubles in plenty of movies. Even from relatively far away shots, you can still tell it's not the actor(s) in question.
Maybe they should have gotten a *real* T-1000 as the stunt double. That way, it could just morph!