Factual error: Throughout this episode, when Carrie talks about the functions of the left vs. right sides of the brain, she has them backwards. The right side of the brain is the creative side, the left is the logical one. Apparently, the writers liked the idea that the "right" side of Carrie's brain would ultimately lead her to do the "right" thing about Mr. Big, but that's not the case. Rational decision are made with the brain's left side. (But then again, Carrie never was very rational when it came to Mr. Big.).
Continuity mistake: When Samantha and her date are in the health food restaurant, she orders wheatgrass shots. The waiter pours them, filling the cups to the brim and then sets one on the table in front of each of them and we see again in a close up that they are both full up. Yet when Samantha picks up hers to drink, the liquid level has gone down a little way.