When Joey is talking to Chandler about giving him money, Chandler in first shot sitting with his legs open, then when the camera cuts back he has his feet up on the table, then when it cuts again his legs are back to their original position.
When Rachel and Monica are talking about Monica "giving her flower" to Roger her fat suit is visible up her sleeve.
At the beginning of the theme song where Monica sits on the sofa, one can see Chandler and Ross talking to the production team/crew.
Dr. Roger says that a hazelnut isn't a nut, but a seed. A hazelnut is a nut.