On the lice episode of As Told By Ginger, Courtney and Miranda are in the locker room. They have just come out of the showers, and they still have their towels on. But, Courtney still has all her makeup on, not even moved at all. She must have waterproof eye shadow and blush!
This is something that happens often in cartoons, obviously they will change the characters clothes for this type of scene but they don't have the character with no make up on just because there is no real reason to change it. If the character often wears make-up it may be an easy way to tell which character it is. This happens in live action movies as well, they should make it more realistic by having no make-up on the character in situations like this. Good catch though!
I don't really think that was a slip-up. The rules that apply to real life don't necessarily apply to cartoons. They are allowed to do things like leave characters' makeup on because they are animated and sometimes characters look alike and makeup is a distinguishing characteristic.
I think that this was NOT a slip-up because in a cartoon, you must know that they don't really care about minor details like that, and you don't know if Courtney ALWAYS looks like that or not.
Perhaps before taking her shower Courtney washed the makeup off of her face and then after her shower she put on some new makeup by doing it in such a way so that her facial apperance would look the same as it was before her shower.