The One With The Mugging - S9-E15
Corrected entry: Phoebe realises that she was the girl who mugged Ross when he was 14. This cannot be true when all throughout the whole show Ross says that he and Monica grew up in Long Island and he moved to NYC for college.
Correction: Phoebe said to Monica (after Ross walked out of the coffee shop) she would mug kids who were coming out of the comic book store near the alley she hung out. Chances are Ross visited the comic store in the city.