Playing the Joker's Carnival Challenge Map as Robin will have The Joker express surprise that Robin "is still alive." Joker is confusing this game's Robin, Tim Drake (the 3rd Robin) for Jason Todd, the 2nd Robin that Joker killed in the comics. Spoiler:Jason Todd appears in Batman: Arkham Knight as the eponymous antagonist, with a reference to Joker torturing, brainwashing and seemingly killing him, making it more odd he'd confuse Tim with Jason.
Contributed by Funland47