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.
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.
When Rachel's new boss is explaining about coffee, his head moves from the front to the side with no time to do so.
Right after Ross is talking about who won the Brown Bird cookie sale, Rachel walks into Central Perk and starts talking about her interview. As she is doing so, her jacket sleeve is already off her right arm and she takes her sleeve off of her left arm. In the next shot, she takes off her right sleeve again even though it had already been taken off.
Brown Bird Charla's beret changes shape between shots.
The plastic canary whistle that Sarah the Brown Bird blows must be filled with water to work. It would leak if it were hung around her neck as shown.
When Ross is at the Brown Bird meeting, the patches on the leader's sash are Boy Scout merit badges.
When Monica, Ross, Chandler and Rachel are at the apartment and Rachel is suggesting how funny Chandler is, as soon as Ross says, "Sweetie, calm down it's going to be OK," a boom mic can be seen for a short period of time, at the top of the screen.