The One with the East German Laundry Detergent - S1-E5
Corrected entry: In this episode, Ross reclaims their washing machine after the Horrible Woman cons Rachel out of it, and Rachel tells him it's brilliant because she "can't even send back soup". But in The One With The Flashback - set before this - Rachel sends back a whole round of drinks.
Correction: When she sends back a round of drinks in the flashback episode, she is a secure, wealthy, engaged woman surrounded by her country-club friends. She might have been more secure to point out other peoples' mistakes as the wealthy, engaged Rach but in TOW the east laundry detergent, she is not wealthy OR engaged and she is constantly guffing up coffee orders. She may be more insecure now.