In the opening scene there's a reading for a heartbeat monitor on the green chalkboard that shows a strong beat, then a flat-line. This is because no one knew if there was going to be a sixth season.
That free $5 footlongs for life Subway Black Card, like the McDonald's Gold Card and Burger King's Crown Card, really exists. These cards are handed out to famous celebrities like athletes and movie stars as part of long term promotional campaigns.
At the start of the episode Abed talks about how they are all in a story. His explanation mirrors series creator Dan Harmon's story structure called the story circle. Abed's breakdown of what's happening line up with the first two pieces of the circle.
When Jeff walks into the study room and sees Britta listening to music, she is listening using the iPod nano bequeathed to her by Pierce in Cooperative Polygraphy (2014), with "songs to take life less serious by" on it.
Starburns, who previously didn't know of the Dave Matthews Band in Basic Intergluteal Numismatics (2014), is now seen listening to the band in celebration.