Pieces of a Stargate can be seen on the walls of Jack Latimer's vault.
Wil Wheaton (Chaos) mentions that to join the team, he requires Parker to dress as Starbuck from Battlestar Galactica (2004) or he would accept Sophie dressing as Deanna Troi. Deanna Troi was a character in Star Trek: The Next Generation (1987) which featured Wil Wheaton.
This season finale sees the last of several long time recurring characters such as Kari Matchett (Maggie Collins) who played Nathan's wife at least once in the previous seasons. Also those fan favorites including; Richard Chamberlain (Archie Leach), Saul Rubinek (Victor Dubenich), Wil Wheaton (Colin Mason/Chaos), Clayne Crawford ( Mr. Quinn). As of Feb. 2022, none of them have returned for Leverage: Redemption.
When Dubenich knocks on Nate's door and says "Knock Knock" they're tying back to Warehouse 13 where the actor was a main character and "Knock Knock" was a running joke on the series.
When Archie tasers Chaos, Chaos's skull is visible for a couple of frames.