Recovery operations used to try and fix the stability issues Xbox Live has been sporadically may have resulted in unlocked achievements relocking, according to site Ripten.
Dan Landis wrote that he had unlocked some of the more difficult achievements in Guitar Hero II, and then noticed recently they had disappeared, so he called Xbox 360 support.
The support person told him, Landis says, that disappearing achievements was a known issue with Halo 3 and "all other games." The person then launched into an explanation about why it was happening:
Basically, while attempting to "fix" Xbox Live, they've systematically been rolling back and recovering portions of their servers to weed out the bugs. Again, this is not the "official" word from Microsoft, but the guy on 1-800-4my-xbox said that's what was going on. He seemed to know what I was talking about when I told him my issue, so that leads me to believe he's not just making stuff up.
I'm a little dubious about this rumour, mostly because we here at Kotaku haven't received a single report about this happening to anyone. I also nosed around in the official Xbox forums and couldn't find anyone complaining about losing an achievement, and you'd think that would be a vocal issue. I've emailed Live Director Larry Hryb about the rumour, but since he's nose deep in CES right now, I don't expect to hear anything back immediately. Once I do I'll make sure to update.
In the meantime, have any of you run into this problem. I don't think I have, but I don't keep that close an eye on my achievements.