I wasn't understanding the whole Ursula thing when Joey and Chandler was in this restaurant and just thought Pheobe was acting goofy. But it was the character who was part of Mad About You who happened to be her sister. Then Jamie and Fran from Mad About you walked in Central Perk and thiought Pheobe was Ursula LOL

I guess it happened because both shows aired back to back on NBC and was set in the same city