Right, and I think Eric is still saying that you'd have to steer clear of that. But the OB line running up to the mando is the thing that compounds the situation. In most other mando situations, you can be short of the mando and to the "wrong" side, but still be able to play your lie with no penalty (other than being in a shitty situation). Here, you can be short of the mando and be OB, whereas another shot could fly on the good side of the mando, and roll backwards to end up at the same spot, but not be OB. If there were an actual river or road there, both discs would be OB, but here it's just an artificial boundary.
I can see the logic behind making the line a mando, but I think it'd just be more confusing for most people. Shot C probably happens in such a low percentage that most people wouldn't question it, and if it did happen, it'd just be one of those oddities.
Of course, there's always the easy solution of taking the OB line away (I promise I wouldn't complain).