Why does this have to be a slip up? Maybe after they slept together she put her pink undies back on. You never know. i noticed this too and never considered it a slip up.
So she's wearing panties. What's wrong with that? Besides, in a television interview, Jennifer Grey (Baby), said that she was wearing the underwear because she didn't want to do a nude scene.
You can view this scene in 2 different ways. First you can always say that she put her underwear on after having sex and they just didn't bother to show it in the movie; or you can assume that Jennifer Gray didn't want to do a nude scene and left her underwear on but was supposed to be nude in the movie and it was an error that you were actually able to see them.