Halo 3 users in Matchmaking discovered a problem. Users were angered to find that their Halo 3 TrueSkill and EXP were not being rewarded properly after each match -- that is to say it didn't work at all. According to Bungie and the Xbox.com support page, the problem is being investigated and hopes to be solved as soon as possible. Bungie noted that the issue may appear, at first, to be an EXP ban but users will be able to tell the difference when a very specific warning message is displayed. X3F will update you as soon as it learns more.