Character mistake: Janet is waiting for Larry to deliver her new car and states to Jack that this is her very first car. But 5 episodes earlier in S8-E13 "Itching for Trouble" when Moose asks if they had a football that needed air, Janet says no, but one of the tires on her car are low.
Revealing mistake: At the end where Janet and Jack are in the bathroom (off camera, as he is confessing why he took the car), - you can plainly see that the door to the bathroom is a breakaway door compared to the other episodes, - and even in that particular episode. It's changed in the middle of the episode.
Suggested correction: It could have been a quick comeback line as a joke. The way Janet was even saying it sounded that way. Also, technically speaking, she did have a car in the past. The trio had bought Roper's Car for a very short time (from an earlier episode) before Roper bought it back from them. Janet never physically drove it but at that time, she was ⅓ owner of it so technically she had a car before.
Yes, I always thought it was a quick comeback.