Continuity mistake: In the episode "Secret Sex," Samantha and Carrie are hanging Carrie's poster in her apartment. In one shot, the whole poster is pinned to the wall and in the next shot, the bottom left part of the poster is curling up. After another shot of Samantha, the poster is pinned perfectly to the wall again.
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.