In the episode where Bart is sent to military school he is taken in the pink car Homer drives, but when Homer, Marge, and Maggie drive away. They drive away in a pink car.
You know, it's possible he made a typo. I have this episode on video (4F21 "The Secret War of Lisa Simpson") and I do notice that they arrive at Rommelwood Military Academy in Homer's pink car but leave in Marge's red station wagon.
Ummmm....so, they dropped him off with the pink car and then drove away with the same pink car? OH MY GOD!!!! THIS IS THE BIGGEST SLIP-UP EVER!!!! Oh, wait, this isn't a slip-up at all and you even admit it.
Hmm, well was it a different shade of pink or something? I doubt it so I think you may have been mistaken about this one. Driving somewhere in a pink car...then leaving in a pink car doesn't really seem like a slip-up to me.
Here's the thing. The arrive in a pink car and leave in a red car. But what probably happened is they went in the pink car, went back home in the pink car, because Lisa wanted to stay and they had to get all her stuff. Because of that, they took them both back in the red car and dropped them off. It all happens during the commercial anyway so we can only assume that's what happened. I don't understand why people need explanations for cartoon slip-ups, but there it is.
Two Slip-Ups in one. Reason #1: You said that Bart get's taken to Military School in a Pink car, so that's one. Reason #Two: You said that the car drives away, yet you said that it was coloured Pink when it was driving away. Thats Two.