This slip up occurs in season 3, episode 4, titled "Boy, Girl, Boy, Girl".

Watch the scene when Miranda and Carrie join the "Goddess Workout" class.

They are waving around some scarves, along with everyone else that is attending the class.

Check out the two ladies in the back, the blonde with the yellow shirt, and the brunette with the pink shirt. Notice their scarves. First the blonde is holding the green scarf and the brunette is holding the black. Keep watching, throughout the scene they switch scarves. Sometimes the blonde has the black while the brunette has the green and visa versa.