Modders have discovered a co-op mode hidden in The Binding Of Isaac: Repentance.
Just ten days after the game was released (thanks, NME), dataminers discovered the launch command "networktest" hidden in the code, and soon realised that it permitted the game to be played in co-op via the game's debug console.
The discovery is particularly interesting given that while there are some workarounds that enable co-operative play, the game is still clearly marked as a singleplayer adventure on Steam.