What a frustrating mess.
AT&T (phone/TV/internet) incorrectly billed me 6 years ago. They sent me a bill for $300, but I had already paid the $300 when I received the bill.
I went through hell to straighten that situation out. It would have been a comedy, except it was so serious. Months of talking to representatives, supervisors, over and over. For whatever reason, even though they would tell me that the problem was straightened out, I kept receiving the bill for $300 every 2 weeks.
After several months of doing battle, I just gave up. I no longer receive the bills, but the $300 is still on my record in my old account with AT&T.
I figure that since next year will be 7 years, the "$300 owed" will hopefully be dropped.