Garry Newman set a trap within Garry's Mod to expose people pirating his work.
"Anyone unable to shade polygon normals?" asked Newman on Twitter.
Hours later he added: "Just enabled this error in GMod today. It happens when you pirated it. Having fun watching people complain."
A search for "unable to shade polygon normals" turns up plenty of results on Google.
There are even pirates asking for help dealing with the error on the official Garry's Mod forum.
The best part about the pirate-catching error code is that the naughty user's Steam ID is posted within it. When they reproduce the code on a forum asking for help, Newman can cross-check the code against legitimate purchases and hoof the pirates out.
Garry's Mod, a physics sandbox, started as a Half-Life 2 mod. The $10 download now works with most Source-powered Valve games, allowing the objects and characters of the world to be spawned, manipulated and played with.