15Calendar Man - Batman: Arkham City
Rocksteady has been known for putting a lot of easter eggs in their games. There was a map of Arkham City in Arkham Asylum as well as numerous teases to Arkham Knight hidden within Arkham City.
Many of these teases were extremely obvious, such as Azrael or the Scarecrow easter egg.
However, the most difficult one to unearth comes from Calendar Man. Every time you visit him, he says a different line. However, if you set your console's clock to December 13th, 2004 (years before Batman Arkham City) ever came out, Calendar Man gives a line about Batman's beginning and end. Considering that Arkham Knight was on its way, that was a chilling thought. The date is also significant as the day Rocksteady was formed.