Continuity mistake: When Carrie is showering after being stood up at her party she is not wearing mascara as she stands with her face under the water, then as she opens the curtain to talk to Charlotte again there is no mascara. She puts her hands briefly over her eyes and when she takes them away, mascara is visible all over her eyes and cheeks.
Revealing mistake: When Charlotte arrives at Carrie's apartment to pick her up, Carrie is in the shower. As Charlotte walks towards the bathroom, there is a clear shot of Carrie showering. Sarah Jessica Parker appears to be wearing some sort of minimal tan colored undergarments, to protect her modesty while filming. (00:24:20)
Plot hole: In the closing scene in Big's car, he asks Carrie how her birthday was, then says, "So how old are you?" She replies, "How old are you?" And they coyly don't reveal their "age box." But they have known each other for 3 years/seasons at this point - dated on/off for 2 years, had an affair, and are just friends now. There is no way they wouldn't know each other's ages. This exchange only existed to benefit the plot of this episode - it makes no sense given how long they've known each other.