I know someone dealing with this shenanigans right now, they sent two bills, which he paid, and then figured it was all sorted.

Many months later, he finds out that they later sent two more bills to the address where he lived in middle school, never called him about it, and then referred them to debt collection because of course he hadn't paid them. He found out about this when the debts showed up on his credit monitoring.

Now they're denying that he gave them his current address, despite the fact that he has multiple other bills with earlier dates where the hospital has printed the correct address on them.

tldr: America's medical billing is an absurd shitshow