Ovo customers report getting energy bills of almost £50,000 after the firm sent massive bills by mistake.
The issue seems to affect customers who were tranferred over from SSE, which Ovo bought in 2020.
One Ovo customer was asked to pay a £49,000 bill for a two-person home - for just four months of energy use, the Guardian reports .
She said: "I was assured my account would be put on hold but am now being chased over a bill of £34,015 for the three months to October.
“What started as one of modern life’s irritations is now taking up many hours of my week and I’ve hit a brick wall trying to sort it out.
I’ve cancelled my direct debit, otherwise they would have just taken the money, but I know that shortly they will put this in hands of a debt collector.”
Ovo has now cancelled her bill and paid compensation for her trouble.
Other Ovo customers also say they've had impossibly high bills from the company.
One said she was billed for £44,800 for two months' power use for a one-bedroom flat.
The woman said: "I’d been asked to send photos of my meters in August as Ovo believed there was an issue.
“I did so and my account went from £600 in credit to £19,000 in debt. Despite Ovo assuring me this was a mistake, the debt rose to over £44,000 in September.”
Her account was later found to be £239 in credit, and Ovo paid compensation.
An Ovo spokesperson said: "We have processed millions of successful [SSE] migrations to date, but we’re aware of a small handful of customers receiving large projections.
"To resolve this, our teams have set up an extra control to check for large bills to make sure we identify them, and fix them, before they’re sent to customers.”
Energy suppliers accused of failing vulnerable customers ahead of winter have been named and shamed by regulator Ofgem.
The watchdog reviewed 17 energy firms in total as part of its review, with each supplier told they need to improve their practices.
There are only 23 energy suppliers still trading, after a wave of them collapsed last year .
Five companies - Good Energy, Outfox, So Energy, Tru Energy and Utilita - were found to have "severe weaknesses".
The report found "moderate weaknesses" at E (Gas & Electricity), Ecotricity, Green Energy UK, Octopus and Shell.
And seven others - British Gas, Bulb, EDF, E.ON, Ovo, Scottish Power and Utility Warehouse - had shown "minor weaknesses".
As part of the review, the firms had to explain how they identify and keep records of vulnerable customers - and whether they had been added to a priority register.
The Priority Services Register makes sure extra help is available to people who need it, including advance notice of power cuts.