[Update: Microsoft claims the issue is now resolved, but we've already had users switch on their systems and claiming it hasn't been. If you've been affected, log out and log in, while hoping for the best!]
Reports around the Web are pouring in from Xbox 360 users who have had their game achievements wiped from existence. People are complaining on the official Xbox forums, and I also received a note from an old friend of mine in England who has suffered the same error.
"Earlier I was playing Mass Effect 3 on Xbox and decide to eat something," reports Louie Falls. "I came back, turned on Xbox and loaded up Mass Effect. I then got a message saying that my Gamertag has been used on a different console and that my game will start from the last time I played on this console. Then I realised that when I got an achievement, my whole gamerscore was reduced to 0. It's happenend to about 50% of the people on my list."
Although Mass Effect 3 has been named in conjunction with a lot of errors, it doesn't seem to be exclusive. Louie says that his friends have also been affected while playing Dawn of War, Halo Reach and Lost Odyssey. He has also called Microsoft's support line, and was informed that Microsoft is aware of the issue, and working on a fix.
Very weird issue indeed, and hopefully Microsoft can tell us what, exactly, went wrong. If you care about your game history, you might want to keep off Xbox Live until this is sorted out.