I had a house where I couldn't switch providers because of an old unpaid bill of £20.
every month a bill arrived addressed to FAO : Abbey National
turned out that three owners before me the owner had died and the house had been reposessed by abbey national.
the fun part was that without the bill being paid the power company wouldn't allow changes to the tariff or allow the change of supplier. And they wouldn't accept payment from me for the bill in question because the bill was in no way my responsibility. (nice little lock-in technique there)
That was 8 years ago, I bet the current owner still gets that bill monthly.