The offers available to the pre-purchasers of Halo 4's map pack DLCs were a little complicated, but the confusion has lead to a free map pack, as long as you grab it quickly enough.

Shortly before the release of Halo 4, Microsoft and 343 Industries announced a promotion that if you played the game within the first two weeks of release, you would receive a code that would give you early access to certain multiplayer specializations. Users who received that code discovered that if they redeemed it, they could download Halo 4's first map pack, the Crimson map pack, for free. This was not something that was supposed to happen, but Microsoft has embraced the mistake and is now encouraging gamers who participated in the, "14 Day, Buy and Play," promotion to grab the maps before Tuesday December 18. Here's the official words from Microsoft:

As an additional reward to those customers that participated in our "14 Day, Buy and Play" program (which ran from Nov. 6 – Nov. 20), we are providing complimentary access to the Crimson Map Pack for a limited time, through Tuesday, December 18 at 9am UTC/GMT.

There were some initial complications with the distribution of the "14 day, Buy and Play" reward tokens, and so we decided to provide the additional reward of complimentary access to the Crimson Map Pack, for a limited time, as a "thank you" to our customers that contributed to the success of the "Halo 4" launch.

[via Kotaku]