What has the energy giant done?
It's been found guilty of breaking energy sales rules between June 2010 and December 2013. Ofgem accused it of misleading customers on the doorstep and over the phone and is making E.on repay about £12m to customers affected.
Who was affected?
For starters there are around 33,000 E.on customers who are normally recipients of the Warm Home Discount. They'll be handed back £35 each.
Anyone else hit?
It seems so. The company has been forced to write to about 465,000 customers it has identified through its redress work, to tell them how to get in touch to find out whether they were mis-sold.
This seems shocking. What has the reaction been?
Which? said: "Our investigations in 2011 found clear evidence that suppliers weren't giving people the best deal, and it's extraordinary that mis-selling has apparently continued for so long since Ofgem began its own investigation.
"Energy suppliers should not wait for the outcome of the proposed competition review and must seize their chance to sort out woeful service standards and put customers first."
Tom Lyon, an energy expert at uSwitch.com, says: "E.on's behaviour is disappointing, particularly given the length of time these poor practices went on for and the fact that they were happening as recently as last December.
"Trust in the industry is already at an all-time low and this fine will only serve to further increase consumers' cynicism towards energy suppliers."
What should I do if I think I may have been mis-sold?
Customers who believe they may have been mis-sold by E.on can call the company's dedicated phone number, 0800 0568 497.