Revealing mistake: In the opening credits for both parts, fat Monica jumps on the couch and it lifts up on Chandler's end. If you look carefully at Chandler, you can see him holding one end of the couch with his hand and pressing up with his foot.
Continuity mistake: Near the start, Chandler is sitting in the coffee house in the big chair. When he's talking to Joey about money, his feet change positions every two seconds. On the floor, crossed, resting on the table, the list goes on and on.