The One With The Triplets - S5-E3
Corrected entry: Monica tells Chandler she is not going to go on the date with the man nurse. As she is walking away, Chandler is going to do a dance and she says don't do the dance. The dance she is referring to is the one he does where he hears that Monica is with someone who is the best sex she's ever had. How can she know about this dance if the first time she sees it isn't until the following episode?
Correction: Easy - Chandler probably does this dance all the time, and she knows him well enough to know that he does it when he wants to celebrate something.