The eponymous creator of Garry’s Mod inserted an error message into his commercial Source engine mod that only appeared in pirated copies of the game.
Gamepron reports that Garry posted on Twitter yesterday asking if anyone was “unable to shade polygon normals”, resulting in posts on the mod’s official forums and elsewhere from those who had been presented with an error message to that effect.
Said message was followed by a long string of numbers in brackets, which it transpired was the user’s Steam ID. With Steam keeping a list of those who had legitimately purchased Garry’s Mod for $9.99, the developer was able to identify and prevent from playing those who had pirated the game, and ban them from the forums while he was at it.
Garry’s Mod, a sandbox physics game based on the Source engine, was first released in 2004 and has been commercially available through Steam since 2006.