In this episode, "Nag Hammadi Is Where They Found the Gnostic Gospels", go to the scene where Liz is driving Luke's truck and is confronted by Lorelai. Lorelai mistakes Liz for Luke and then Liz remarks, "I should have put lipstick on," or something like that. But you can clearly she is wearing some sort of lip color and it is fairly heavy. It could have been a sarcastic comment, but I would consider it a slip up.