Are there any shows that had a common theme for maybe just one episode? I remember when Sabrina's cat Salem crossed over to Boy Meets World, and the rest of the TGIF lineup for a night, all the shows that night had a common theme.

Are there any other shows that have done this?

I think NBC Saturday had a Hurricane theme night.

NBC had a "blackout" Must See TV night back in 94.
3 of the 4 sitcoms featured blackout stories, Seinfeld did not.

Earlier in the week, Frasier had a blackout episode as well.