In the flashback where Joey gets the turkey stuck on his head, Joey complains that it smells bad and Phoebe replies, "Of course it smells bad, you have your head up a turkey's ass." But in the original episode with that scene, The One With All the Thanksgivings (season 5, episode 8), Phoebe replied, "Of course it smells bad, you have your head in a dead animal."
While Rachel and Phoebe are spying Chander in the Central Perk and talking about movies, Rachel's hands are empty, but in the next shot she's holding something.
When Phoebe and Rachel are on the phone to Chandler in Central Perk, the car he is in outside continually disappears and reappears depending on the angle from inside the coffee house.
In the first scene, right after Monica says, "I could show this cake a good time," Emma waves and smiles to someone behind the cameras.
When Chandler gets the phone call that their offer on the house was accepted, a large light is visible moving in the reflection of the painting behind him.
In the shot, right after Joey says: "I knew he couldn't be with a woman for 45 minutes", a microphone is visible above Phoebe.
Ross says he can see Chandler and Monica having dinner but as established in previous episodes, this is impossible. However, he may have seen them eating by the window.
While arguing with Monica and Chandler before the clipshow, Rachel wants to call Monica by her name, but accidentally calls her Rachel instead.