Garry Newman, creator of Garry's Mod (known commonly as GMod), doesn't like it when people pirate his Source-Engine sandbox-style playground. So he threw in some code that would give pirates a specific error when they played the game. But when they went to report their bug, they ended up giving themselves away.
GMod was first released in 2004 as a free mod, only to be released as a paid Source-engine mod in 2006. It currently costs $10 on Steam in the US, but has been on sale for $5 several times in the last year.