I can't remember exactly what happened but I think a friend of mine managed to have an achievement missing from his gamertag. He'd unlocked a licence on Burnout Paradise but I don't think he was connected to the internet. Next time he used his gamertag he'd recovered it on my console and earned some achievements for another game, and when he recovered it on his again he still had the savedata, obviously, but the achievements from before were missing.
I imagine that could happen if you're giving your gamertag details away to people.
I really couldn't care about achievement 'cheating' unless it interferes with other people's gameplay. Seriously, who cares? Well, I mean, MS obviously do but that's just sad.