This must be the strangest bit of gaming voodoo I have heard of.
With the Call of Duty: Modern Warfare 2 retail game or demo a glitch has been reported that unlocks the full versions of five downloadable games from the Xbox Live Arcade service. This glitch has been unlocking games such as Metal Slug XX, Garou: Mark of the Wolves, The King of Fighters ’98, Ultimate Match, Metal Slug 3 and Samurai Showdown II. It is reported that you can open and play the said games while in Modern Warfare 2, but can’t save the games.
When this glitch hits, all the affected titles are classics from legendary Japanese developer SNK which is not even a game from Activision or Infinity Ward, it is in fact from a completely unrelated Japanese studio.
I am sure this won’t be around for long, so if you come across it, take advantage.