Continuity mistake: When Phoebe says that they should not tell Ross that the date is cancelled so that he gets stood up, Joey says, "I hear that's bad" - implying that he has never been stood up so doesn't know how it feels. However, in TOW Two Parts (Part II), Ursula stands Joey up, and he takes it pretty bad.
Continuity mistake: When Phoebe and Joey are at the coffee house discussing their plan to ruin Ross' blind date just before Joey says, "Ooh I hear that's bad" the pendant on Phoebe's necklace is smooth and blue. When the camera pans to Phoebe after Joey says his line the pendant is flipped over and reveals a new design. This continues to happen throughout the scene.