Millions of families are being left up to £400 out of pocket due to errors in calculating their household bills.
A new report claims 70% of consumers have been overcharged in the past year, with a third more than once.
Billing mistakes by firms responsible for essential bills, such as utilities, telecoms and mortgages totalled around £6.7bn in 2012, according to figures from price comparison website uSwitch.com.
It found customers were overcharged an average of £196 and had to wait around 53 days for a refund, with one in 10 overcharged by £400 or more.
More than one in ten (13%) had to wait between two to six months to get their money back. Some (12%) are still trying to get the issue resolved, while 6% were never refunded.
The most common causes of the errors included charges being added to a bill which never should have been (42%), incorrect tariff or product details being used (32%) or a special offer or discount not being applied to the bill (25%).
Some 95% of the mistakes were spotted by the customers themselves - not the companies.
And researchers found that a quarter of people received a bill that simply did not add up.
On average people spent eight hours and £23 on phone calls and correspondence trying to sort out mistakes, with just 7% receiving an instant refund.
Ann Robinson, director of consumer policy at uSwitch.com, said: "Overcharging on household bills is rife and yet it still seems to be down to the customer to spot it.
"Consumers have to keep their wits about them and ensure that they check all their household bills carefully - if you are not checking then the chances are that a mistake will have slipped through and this could be costing you dear.
"At the same time I would urge companies to do as much as possible to help their customers by making household bills simpler, clearer and easier to understand."
She added: "Nobody can afford to be left out of pocket because a company didn't quite get its sums right."
[Find the cheapest gas and electricity tariff for you]