The guy who played Frasier Dad (John Mahoney), was also on Cheers, but not as Frasier's dad. He played "Sy Flembeck" in episode: "Do Not Forsake Me, O' My Postman" (episode # 11.5) back on October 29, 1992. I find it funny. O well.
Also, since I'm talking about Frasier, here is the list of people who did the most guest apperances.
contemplative