There's a post on the IW forums saying that they played a group of 10th prestiges on Hardcore S&D. He said whenever they killed one of the 10th prestiges they would lose xp. This resulted in them going back to level 2.
Here is a video for proof.
YouTube- MW2 ranks reset by 10th prestige
I don't think they are lying because how does a level 5 have models and other guns that aren't unlocked at level 5. Other posters said this has happened to them. Just a head's up,watch out.