An out-of-the-blue energy bill for hundreds or thousands of pounds can land on your doorstep with a sickening and shocking thud. But just because your energy supplier sends you a charge for what it says are past underpayments, do you actually have to pay?
It happened to Independent columnist Terence Blacker last week. He was suddenly confronted with a confusing demand for almost £6,000 by his energy firm, E.ON. After he wrote about it in the paper the company stepped in to sort things out.
"It was all a terrible mistake, apparently, Mr Blacker said. "'Human error' was the mantra of the moment."
He was able to cut the back-bill to less than a tenth of the original amount, but said: "It's worrying that vulnerable people may get similar demands and not know what to do."
That's exactly what happened to the parents of Stewart Graham. The Birmingham couple in their mid-70s had a surprise demand for £1,200 from EDF which left them fearful of turning on their heating because of the cost.
We passed on the problem to EDF and, to its credit, the supplier appears to be sorting things out for the couple to investigate whether the surprise bill is due to a faulty meter.
"We will, of course reimburse the customer without delay if the test does reveal a fault and we will not send any bills to the couple until the results are confirmed," EDF said.
But what if you haven't got the power of the press to help? The Citizens Advice chief executive, Gillian Guy, said: "It's a real shock – and very confusing - if you get a big energy bill out of the blue saying you owe money for gas and electricity when you've regularly been paying bills. This can happen if your bills have been based on estimated usage or if the energy company has made a mistake."
But it seems to be happening more and more, she warned.
"Our consumer service has handled more than 2,000 problems with back-billing since April, making it the fourth-biggest energy issue dealt with by the consumer service."
The rule about back-billing is simple: you are obliged to pay for energy you have used but suppliers must follow certain rules about how far back they can bill you.
In short, they are not allowed to charge you for energy used more than 12 months ago if "the supplier was at fault for not having sent a bill to the customer". But the time limit only applies if you have made efforts to contact your supplier or provide them with correct meter readings.
"It is important for customers to know that, depending on the circumstances, you might not have to pay the full bill, for example if you've not received a bill or request for a meter reading but have been asking your energy company to send one," said Ms Guy. "And if you can't afford to pay the catch-up bill suppliers should offer you a payment plan that allows you to repay any debt over the same length of time that it has built up."
Meanwhile, The Independent and Policy Review Intelligence is hosting a Fair Energy Summit on Monday, supported by British Gas, Co-operative Energy and E.ON.
Energy company bosses and energy minister, Ed Davey, will be quizzed about rising prices.
Readers are invited to send in their questions and comments to email@example.com as well as contribute during a live internet broadcast at independent.co.uk at 10am on Monday.