Knightmares - S2-E11
Revealing mistake: When Michael and KITT are going to jump at the dam in order to save Cara from drowning in the water container just before they jump you can see a stunt ramp below KITT.
A Knight in Shining Armor - S2-E13
Revealing mistake: When Michael fights the bad guy at the end of the episode, it's easy to tell that they're both stunt doubles.
Revealing mistake: The shot of KITT releasing his oil slick at the end of the episode is backwards: "KNIGHT" appears as though it's in a mirror.
Answer: I would say that unless there's visual evidence it was another K.I.T.T. car, as in you could see the bar scanner or cut steering wheel, people on here would correct the mistake by saying more than one 1982 Pontiac Firebird Trans-Am was sold and there's no reason it's meant to be a random car.
Bishop73