This is most assuredly a glitch, but if you log on to Xbox.com right now and look at Fable 2 in Games on Demand you will see that it is selling for the rock bottom price of free.
We’ve seen things like this happen in the Marketplace from time to time. Like when you could download the golden Hammerburst for Gears of War 2, or Halo Reach for 9999999 MSP. I’ve added it to my queue. Get it while it’s hot.
UPDATE: This has been fixed. Fable 2 now costs on both the website and the dashboard.