This slip-up is in the episode where Joey dates Phoebes' twin sister. If you look closely at the outside of the resturaunt where Ursula works, Riff's, you can actually see that the sign that says Riff's is being held in front of the camera! It's really obvious. The best time to see it is the second time it shows the outside of the resturaunt, and a guy walks in front of the camera, between it and the sign. I think someone should have caught that!