That code enables buyers to download a small 108KB file from the Xbox Live Marketplace, which unlocks three pieces of extra content--the Hall of the Dead dungeon, Wreckager legendary cutlass weapon, and Halo-themed Spartan armor and energy sword--that are stored on the game disc but cannot be accessed otherwise.

Microsoft claims that the blunder only affects "a small fraction of the total number of Fable II games shipped and sold," with the company noting that it is "investigating the scope of this situation" and "will have a means to deliver replacement codes by Oct 28."

Separate reports about the missing insert popped up on Joystiq and Kotaku. Cases that display an orange "48 Hours of Live" sticker supposedly pack the card.

It's the latest mishap for the Fable II Collector's Edition, which lost a number of promised goodies a few weeks before its release, though the price was lowered.

Developed by Lionhead, Fable II arrives in stores today. The Xbox 360-only open-world action-RPG is available in a regular $60 version and $70 Collector's Edition.