Continuity mistake: In the scene where Monica and Ross discuss how Emily has decided to call off the wedding after Monica suggests they postpone the wedding, Monica is wearing a black watch on her left wrist. This disappears and re-appears a number of times throughout the scene.
Continuity mistake: As Ross says "this is our wedding day", in a close-up Elliot Gould is holding a bit of paper in his left hand (glasses in his other hand, out of shot). In the wide shot the paper is in his right hand, then jumps back to his left when the angle changes.
Continuity mistake: In the episode where Rachel goes to tell Ross she loves him (in England) she leaves her room with a brown sleeveless vest and a cardigan over her shoulders. The cardigan then disappears in all other shots until after Phoebe says "he's in love with the British chippy". Her leather jacket also goes from being in her hands to over her arm.