We're getting reports from readers that Fable III is currently available for free on Xbox Live.
To start the download, head to the game's Xbox.com listing, click on download, go to use Microsoft Points to complete the purchase then finish the transaction. The game should then start downloading to your Xbox 360 console.
Seems a bizarre mistake to pop up now, of all times. Wonder if it's related to an impending Fable announcement for the Xbox One? I mean, it's not like this is the first time a Fable game has mysteriously done this, and the last time was right before the launch of Fable III...
[Update] Microsoft has since made the game free to download without the above method described, so we assume this is intentional.