Punk Chick - S1-E22
Other mistake: Jackie and Kitty are baking a pie in the kitchen and Kelso walks in. Jackie and Kelso have an awkward encounter as if they are separated, but they got back together at prom.
Other mistake: At the beginning of the episode, when the mermaid goes underwater, she holds her breath.
Other mistake: When Carrie, Samantha and Miranda step outside of the hotel so Carrie can have a cigarette, she takes a couple of draws then throws it down on the step. She tries to stub it out with her shoe by twisting her foot on it but she (the actress) accidentally kicks the cigarette down the step, so she has to just pretend to still be stubbing it out.