When Abed is talking to Britta at his locker, you can see the fake beard worn by Evil Abed in previous episodes.
First episode where the Greendale Community College parking lot isn't in an open lot. It has now become a parking structure.
Brian Van Holt (Willy) previously starred in one of Abed's favorite shows, Cougar Town (2009). In Cougar Town's series finale, Van Holt appeared via iPad video chat.
The Dean references an anti-bullying rule that applies specifically to him. This rule was revealed in a newspaper clipping in Queer Studies & Advanced Waxing (2015).
Instead of giving Jeff a pay raise , the Dean offers to make Jeff a Professor Emeritus. 'Emeritus' simply denotes 'retired'; it is not some indicator of higher status, not some special award, it just means you are a retired professor.