This is a bit of an odd one and a serious software flaw in the Xbox 360 that Microsoft better patch quickly.
According to reports the latest MW3 patch could accidently clear out some of your achievements and gamerscore and while most of your gamerscore should be salvageable it’s not a guarantee and neither are your achievements.
The bug appears to centre around the Xbox 360’s ability to migrate profiles between Xbox’s but there is a bug with this patch where it looks for your old data and if you don’t have any stored it simply imports blank data and overwrites what you have on the Xbox.
And with that all your achievements and gamerscore for ALL games is wiped.
You can get most of it back by signing out of Xbox Live, signing back in and if it’s still a problem then re-download your profile.
However this can result in your latest achievements simply being wiped.
My advice, don’t play MW3 this week and by that time this should in all likely hood be patched. Or play it on the PS3 instead
Last Updated: March 14, 2012