I recently submitted an unobtainable flag for one of the game's achievements. I also contacted the publisher (who also worked with the console ports) about the achievement issue, as well other game breaking issues (crashes during leaderboard calculations).
I got a response back today, which reads....
Thanks for letting us know about the achievement - we weren't aware of
this bug so we'll take a look on our end on getting that working.
Regarding the high scores, we are aware of this one - we've been working
with both our middleware provider and Xbox themselves to fix this,
because the bug does not occur in the test environment, and also doesn't
seem to occur in Xbox's own testing environment, nor on other consoles.
It's made testing it a bit of a headache to find a fix for. We are
hoping to locate something soon which is actually causing this.
I will be keeping an eye out for any updates and will post any findings here.
Fingers crossed...