The Mantis-Eye Experiment

Mission Creep has numerous items that either appeared in previous episodes or belong to characters seen or named in previous episodes. Among them:

Season 1, Episode 9: "Ice Station Impossible"
Nat King Cobra's Venomator. Nat King Cobra is never seen in the series, however his henchmen fight Race Bannon at the beginning of the episode in an attempt to steal the Goliath Serum. Bannon thwarts them but ends up getting killed in the process. The serum of course ends up at the Venture Compound.

Season 1, Episode 10: "Are You There God, It's Me, Dean"
Grover Cleveland's Presidential Time Machine is briefly mentioned in the cold open for this season one episode, but then never seen again in the series until now.

Season 2, Episode 1: "Powerless in the Face of Death"
Mr. Monday's Caledar of Magic Mondays had not been previously seen in the series, but Mr. Monday was featured as on of the prisoners jailed along with the Monarch in the season two premiere. He made a second short appearance in the first part of the season two finale, "Showdown at Cremation Creek, Part I" when he was killed by Phantom Limb along with a few other inmates locked up with the Monarch.

Season 4, Episode 9: "The Diving Bell Vs. The Butter Glider"
As the Monarch notes, Mission Creep is riding his old Butter Glider, which the Monarch had an... let's just say unhealthy relationship with, back in this season four episode.

Season 5, Episode 6: "Momma's Boys"
Maybe Man's Indifference Engine, belonging to Maybe Man, seen as one of the inhabitants of the Dunwitch Asylum. Radical Left, who is now a member of the Guild council, was in the asylum with Maybe Man (among others) but escaped.