The Ted McGinley Curse?
I was browsing through http://www.popcultureaddict.com/ after seeing a link to it on cult_tv_lounge. Some interesting stuff there, including Defending Dawn, A Visit with Neal Adams, Who Cares? and Gothic Romance. However what really caught my eye was THE CURSE OF TED MCGINLEY.
It seems that Ted McGinley has a history of being a "ringer" in sitcoms that are about to die horrible and slow deaths. He turned up on Happy Days as a "replacement Ritchie" and later on Married with Children in a similar sort of role. It seems then that his curse involves damning a TV show by introducing new characters. And it's not just that site that thinks so. Jump the Shark (named after an episode of Happy Days where the Fonz jumps a Shark) has a whole Ted McGinley page.
Now here's the weird thing. I probably watched a good number of the shows that he's appeared in, but I can't remember him at all. Zero charisma or what? Not quite as good as being Kevin Bacon (with his connections), is it?