Continuity mistake: When Ross, Rachel, and Amy arrive at Monica and Chandler's apartment the table is set, and there are folded napkins in napkin rings on the plates at each place setting, but after Amy insults Joey all the napkins have vanished.
Revealing mistake: When Chandler grabs his bag he accidentally lets the box full of wedding china fall off the stool. Watch closely (play in slow-motion). The box falls on the floor and shattered china falls out. The china was ALREADY broken before it falls on the ground. (00:23:00)