A retroactive example - when Bobby D is Put on a Bus in season 3, it's explained that he's on a task force in Chicago. Todd Williams subsequently took a role on The Chicago Code.
Inverted: In 2x03, "A Stand-Up Triple," the very non-maternal Mary asks Stan and Marshall to shoot her in the head if she ever has three kids. Two years later, the actress was pregnant with her third child.
The Danza: Mary Shannon, played by Mary McCormack. Incidentally, the alias the character uses is "Mary Shepherd".
And in season 3, Allison Janney plays new WitSec director Allison Pearson, and Raph's aunt Rita Molena is played by Rita Moreno.
Hey, It's That Guy!: Stan is played by Chris Rock's maybe-racist teacher. Mary and Marshall themselves have had several secondary roles in various series and films.
Mary is Mrs. Howard Stern. And when Mary got shot, the lady that she was protecting is played by Angelica Castro, or Mrs. Cristian De La Fuente. (Mary, he's married!!!)
Also, Stan aka Jose Quesada is back to being himself lately.