Cheap gas customers could check their bills and perhaps look around for other tariffs if the one they are on is costing too much.
The Derby Telegraph reports that one customer, a 94-year-old man, was sent a gas bill despite not being connected to a supply for six years.
Kenneth Kirkman receives his cheap electricity from Scottish and Southern Energy but the energy supplier also billed him £143 for gas that he had not used.
He had been in hospital when the bill was sent and returned to a letter threatening him with legal action if he did not pay.
A spokesman for the power company said the mistake had happened because it sent an estimated reading.
"We have spoken to Mr Kirkman's neighbour and apologised and explained that we will credit his account and also write to him explaining this," the representative told the newspaper.
"We will also make sure this does not happen again in the future."
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