Continuity mistake: At the start of the scene in which Ross breaks the little girl's leg, we can see that the stairs are just separated by railings. Later in that scene, when Ross and Chandler start walking up the stairs, there are walls on both sides of them.
Continuity mistake: The gang enters Monica's apartment and it is full of dead Christmas trees. After everybody has entered the apartment we see that the apartment door is shut; the shot then changes to the gang around the sofa, but in the next shot when Rachel answers the phone we can see behind her that the door is open.
Continuity mistake: In the classroom Ross has his head down because he didn't want to be called on about the number of boxes he sold. The mother brown bird walks behind him and her right hand is on her hip while she is clearing her throat. In the next shot her right arm is pointing upward.