You get low-hundreds (not an exaggeration) different documents, handed to you and via mail over the course of a year (9ish months + a few after, ends up close to a year), some of which are bills and some of which look like bills but say "this is not a bill" (so... why'd you send it?) and some of which are from your insurance referencing other things and blah blah blah, then you get a few that are clearly a screwup and you call the provider and they're all "LOL no your insurance got it it's fine, ignore that bill" (!?!?! seriously, dafuq, how many people just pay it and do you give them their money back unless they ask?) and of course one of those pieces of paper is gonna slip through the cracks.
Insurances companies get a lot of shit, but the whole medical billing complex is rotten, top to bottom.
Reason #283942 that our medical system is horrendously broken in the US.