One of the best selling points of a standard plan G is you pay your $240 and any bills after that "ain't your problem, period." Anybody tries to bill you, you say "I thought you accepted Medicare assignment", they say "we do", then you say "well you better sharpen your pencil on those claims because I already paid my $240 and I have a plan G super payer". It's almost as good with plan N, but you need to pay the $20 for each doctor visit. That, as opposed to an HD plan, where you're paying all kinds of stuff, and who knows if they've got the claims complete. Anything they don't file right will be shown as your responsibility, even though it's because they didn't file it right.
I think most of us have seen the enormous difference in price between what they bill and what Medicare pays. So they "make a mistake" and leave off one service of a 10 service claim. So say the billed amount on that forgotten service is $150 and the Medicare amount is $20. But they "forget" to file it, so along with your 20% that you're paying on the other 9 (working on your Plan-G High Deductible), they toss-in the $150, and hope you don't notice.
Hospitals have a ton of money flowing through, but the margins are narrow. I'm not saying I know for a fact they do this on purpose, but I've seen a lot of these mistakes, and didn't get any mistakes where the patient got the benefit. It almost certainly depends on how shady the corporate hospital chain is... I'm sure some are better than others.