Strange "AimTarget_GetTagPos" error in MW2

Though not helpful, Some assumptions can be made:

AimTarget_GetTagPos - Likely this is refering to the hitbox (or target area) that you are currently targeting (with your reticle)

tag[j_helmet] - I'm out on a limb here, but I'm guessing this is refering to the helmet area of a person. Now why this is importaint is that MW2 likely calculates damage different if you're hit in the helmet area as opposed to the face area (or whatever they call it)

So the full error in plain english would read something like:

Game could not find the helmet on your current target

As to the implications of why this happened. I'm going to say that somebody is running some kind of hack (or mod) that is masking their avatar's head, thus preventing headshots. Or just a poorly put together hack that failed to cover some system check.

tl:dr

MW2 is constantly being hacked, so it's likely some leftover junk from one of those. Reset your xbox and it shouldn't be a problem (unless you pick up the hack again)