This slip up occurs during season 3, episode 2, titled "Politically Erect".

Just after Carrie talks about her boyfriend's campaign and she waits for him after every speech, they make their way to her apartment. They start kissing and he carries her into the bedroom. Then you see Carrie, topless, obviously having sexual intercourse with her boyfriend, but as the sheet drops a little you can CLEARLY see that she is wearing pink underwear.