I don't think they're usually forgotten. I think they're most likely mistakes. Medical facilities are stunningly incompetent when it comes to billing.

Billing is incredibly complex, especially when insurance is involved. There only 68,000 ICD-10 codes, which will be what almost every care provider uses in billing. Providers notes supplement these (sometimes), but those aren’t always complete or accurate, and because every encounter has some unique elements it defies rigid finite classification.

I was referring to their seeming inability to track or resolve payment issues in a reasonable amount of time.