Visible crew/equipment: When Monica is waving around her shellfish hairdo in the final scene at the hotel, the camera zooms out. When it does this, look at the top of the set. Not only can you see where the set ends, but you can also see the stagelights, hanging equipment etc.
Factual error: The main part of the episode is played out in Chandler & Monica's hotel room (which is situated in between Rachel's and Ross's rooms). The window is opposite the audience and the door on the right. However whenever we see inside Ross's hotel room, he has a window opposite the audience as well. This is impossible as this should be where the hallway is to get to the main door to Chandler & Monica's room. Also, throughout the episode Monica, Chandler & Phoebe all listen through the walls (on either side of their room) to what the other friends are saying, meaning there must be no gap between Ross's and Chandler & Monica's rooms- making the window in his room geographically impossible.
Factual error: There is an impossibility with the hotel rooms in Barbados. Ross gets from Monica and Chandler's room to Rachel's room far too quickly for it to be real. Also, when the camera moves between rooms (cutting through the wall), you can see that both Rachel and Ross' rooms are too long. If the walls were where they actually are, they would edge through into Monica and Chandler's room.
Continuity mistake: Chandler, Monica and Phoebe are shown listening at the wall between their room and the room Ross is in with Charlie. However, when Ross comes into the room, the outside hallway is shown, and it is clear that the wall they were listening at actually connects to the hallway, not another room. Ross even walks in that direction when he leaves, proving it's not a dead end or similar.