Continuity mistake: In the one where Ross finds out he's having a boy (series 1), just before Rachel lets it slip, she is sitting on the edge of a chair eating a cookie. Watch her dress, it changes back and forth from being undone from the waist up, to only having two buttons open at the top.
Revealing mistake: When Emma starts crying and Rachel picks her up, look at how much the baby is moving when she takes it out of the cot. In the next shot it's suddenly only moving it's head, and very slowly at that. This is because it's a robot - and it's very obvious.
Continuity mistake: There is a long shot of Rachel and another person sitting on the sofa and Rachel's wine glass is almost empty. She then goes over to the the counter to fill it up. However, before she starts to fill it, a significant quantity of wine can be seen in the glass.
Continuity mistake: In this scene in Central Perk, Monica tells Chandler she has hired a stripper for him as she feels bad for not letting him have a bachelor party, and tells him to think of it as a 2 month anniversary present. However this episode is set 2 episodes after the Halloween episode, therefore allowing us to assume it is set sometime in November or later. Chandler and Monica got married in May, as stated several times in the show by the cast, therefore they have been married for around 6 months or more, not 2 months.
Continuity mistake: After Rachel explains her plan for the fake party, just as Emily opens the apartment door the message written on the Magna Doodle reads "Joey enjoys the," but when Emily and Ross are standing in front of the door the message reads "Joey eats little."
Continuity mistake: After Rachel spins the bottle and it points to Joshua, when she crawls toward him on her hands and knees the bottle is in front of her knees in the shots facing her, but in the shot facing Joshua the bottle is at her side, near her foot.