I know that some tv stars that do more than 1 tv series will sometimes make comments about their former tv characters in their new tv series, such as sussane somers once made a comment about chrissy snow in step by step infact she even did the chrissy chuckle laugh, i remember michael j fox in the last episode of spin city he did, he made a comment about alex p keaton his family ties character, but ive never known anyone to take it as far as john ritter on 8 simple rules they did come and knock on our door about threes company, ive even seen where some tv series will do storys about popular tv shows, i remember whats happening did their own version of happy days and u guessed it threes company, where wayne played chrissy now that was a gut buster, and rosanne did a show about gilligans island and so did baywatch, so my question is was there any other tv stars that made comments about their former characters in their new show?

Yeah on What I Like About You,they did a Beverly Hills 90210 skit

And the refernece back to the "The Bob Newhart Show" in the series finale for the "Newhart" show.