It seems those who took Microsoft's advice and downloaded Halo 4's Crimson Map Pack for free (as part of 'thank you' to players) are being banned from certain portions of the game. Halo 4 players are flocking to the game's official forums to report having downloaded the map pack and finding their accounts banned for certain parts of Halo 4's online features.
343 Industries and Microsoft released a generic statement, assuring fans that they are "currently investigating the recent reports of permanent matchmaking bans."
They told fans more information would be provided today, but so far we've not yet heard anything. A forum moderator did insist that they are "not permanently banning players for previewing or purchasing the Crimson Map Pack."
I don't think a ban from Halo 4 was the thanks players were looking for. We'll continue to keep you updated along with the official ban thread.