I love it when a casting director pulls in actors from a single show. For example, the last episode of Bones had an actor from the American version of Queer as Folk - and this episode has another actor from Queer as Folk. Love it! In an earlier episode, two actors from the show Veronica Mars appeared together.
And then, in Warehouse 13, they regularly pulled actors from SyFy shows such as Eureka (aside from how the character Fargo goes between both series) and a few other shows that I'm forgetting at the moment.
I love it, especially when the actors I miss from shows that I loved reappear again!