Revealing mistake: When Monica gets out of her room to show Chandler her sexy clothing, as she opens the room's door, the buildings from across the street move. The scenario was a curtain and the wind that the door made when she opened it on the set made those curtains with the buildings printed in move.
Continuity mistake: In the one where Chandler watches the video of a woman giving birth, he takes a video out of the VCR and places it on the table (he's staring at the box and when Monica moves it we hear that a tape's inside). When Monica enters the room, she asks him what he is so upset about and he presses play on the remote control. The tape immediately starts playing, despite no-one moving to put it in the player.