Revealing mistake: Whenever Monica's apartment is shot from the point of view of the balcony looking in, fake walls have to be put up across the room to cover the open space (since there is no wall there usually). When the friends are looking out the window at Ugly Naked Guy and his girlfriend in this episode, you can see that the rear wall does not extend nearly far enough - there is a large black space behind Monica.
Revealing mistake: After Phoebe tells Rachel that Paolo made a pass at her, we see a shot of the outside of Monica's apartment block and Rachel is throwing Paolo's clothes out of the window. Look closely and you can see the clothes have been very badly superimposed on to the background.
Revealing mistake: When Rachel is doing the shuffling trick, it is clear that it is a trick deck. The cards, whilst flying from one hand to the other, change the angle of flight in mid-air, and some of them even move backwards as she moves her hands further apart from each other. This could only happen if the cards were connected to each other.
Revealing mistake: When Phoebe says something like, "Look, the last candle is about to go out", she starts counting down, saying "-47,-48..." and you can see Monica looking at the candle. But there is a light source behind her (it can't be a candle because she is looking at the last candle).
Revealing mistake: When Rachel first meets Paolo she's looking for the cat in candlelight, holding the candle in front of her. You can see her shadow on the wall, as well as the shadow of the candle, revealing that the light is not coming from the candle.
00:07:25 - 00:14:30
Revealing mistake: In the scene where they are in the coffee shop talking about Rachel's dream about Chandler. Rachel is carrying a coffee pot around then sits down. When she sits she puts her hand under the coffee pot, which would be extremely hot if it were real.