Hi. I'm very sorry for bothering you with this issue, as there probably is a FAQ section somewhere that answers this, but I haven't been able to find it. This is my situation:
I've been going through all the Call of Duty games, getting all achievements (just recently completed MW3), except from that last one on Call of Duty: World at War that requires you to reach 10th prestige online. As I've completed all achievements for the other games I finally decided to go for this achievement just to have the CoD series 100%. So I put in the disc, and checked my stats: 1st prestige, lvl 35, so I figured it would be a long run, but I was willing to do it (after all I had gone through the pain of getting the general achievement for CoD3). I then decided to check the exact description of said achievement, but when I did it appeared that it had already unlocked!!!!!! This I discovered today.
I had no clue what the heck had happened, I obviously hadn't reached the requirements for the achievement, so I went and googled it, and sure enough I think I found the answer.
Sometime last year I remember getting a game invite from a friend (he told me through MSN that he had sent me a game invite, because he knows I have my notifications turned off most of the time) who said he was in this awesome zombie lobby. He didn't explain to me what it was, but since he is a good friend of mine I decided to join it. As soon as I joined the game text flew all over the screen in all sorts of colours and I couldn't see any zombies, so I decided, after explaining to my friend that I didn't find this any amuzing, to leave the game. At no point did I communicate with the 3rd player, whom I now understand must have been the modder. The reason why I have not discovered this until now is that, as I've explained, I usually play with my notifications off (I don't like random messages/logins to interupt my gaming), so I couldn't see the achievement pop. And since the achievement rewards you with no gamerscore at all (0), I couldn't possibly notice it from looking at my gamerscore for the game.
I'm now scared to death that my gamerscore might get zeroed out, or that my xbox gets banned, so I am literally begging anyone to please do something with this. I've been working so long with this account, and I really don't know what I would do if something like that (getting banned etc.) would happen to me. Again, please help me, I have no idea what to do, except from what I am doing now; trying to contact a moderator on the forums.
I appologize for my bad English. I'm from Norway.