In the club scene, Frank comes in singing "I still got the moves" to which Mac replies "you're acting like it's 2006." This is a callback to Mac Bangs Dennis' Mom (2006).
Charlie and Dee also kiss in S03E04 'The Gang Gets Held Hostage'.
Although the gang obviously live in Philadelphia the outside shot of the Diner Dee and Charlie have lunch in is the Empress Diner in East Meadow Long Island in New York.