Problem: On my Xbox I canceled a content download (like the Gears of War Flashback Map Pack). Now the download doesn't show in my history and my redemption code shows as invalid when I try to resubmit it.
Solution: Log on to your Xbox Live account's Purchase History page: http://marketplace.xbox.com/en-US/myAccount/PurchaseHistory.aspx. Find the purchased content (in my case the "Flashback Multiplayer Map Pack") in your history and click "Add to Queue." You'll be able to download it now.