The developers collaborated with the folks behind the YouTube series Marble Hornets.
When a glitch in the game let players escape the game boundary and be invulnerable to Slender and his Proxy, the developers noticed this error and if the player tried to repeat the glitch they would fall through the ground and see a disturbing image of Slender-Man with the caption "Not even a bug in this game can save you from me", thus forcing players to play the game properly.