The One Where Chandler Can't Remember Which Sister - S3-E11
Corrected entry: When Rachel is eating at the diner and she and Monica are talking, there is a red glass sitting in front of her. In the next shot she is talking to Mark and says "It's not funny!" and the glass is gone; in the shot after that it's back again.
Correction: When Rachel and Mark are talking, Monica picks up a pitcher of water and you can see her reach for Rachel's glass. So, she is refilling it, that's why it's not there for a while.