The problem comes from the constant regurgitation of older news articles and people cutting and pasting from old articles and sometimes forgetting to change something or making a mistake about what they are cutting and pasting. And other publications cutting and pasting bits from other publications.
So for this story - it's all based on quotes from a royal reporter Robert Jobson who had written a book about Diana, and more recently a book about William turning 40.
In promoting the most recent book he did an article talking about William's tendency to have a short fuse like Diana.
He used the phrase: She (Diana) could be your best friend one minute and the next your worst enemy.
The first article he was mainly talking about William and also mentioning that he'd heard from a courtier that Kate could give as good as she got when it came to arguments but in general she was the calming influence.
So then in a different article later Robert Jobson is asked to comment on Harry and William's moods. He just regurgitates his quote about Diana 'best friend one minute and the next your worst enemy.' and now applies it to Harry as well as William.
Both these different articles are clipped and republished by lots of different publications after the initial articles were published in the DM.
What probably happened at the Mirror is that whoever was tasked with repurposing the second Mail article about Harry's moods, had been using multiple archive articles to copy and paste different Jobson quotes and had got confused someone about all the different people accused of being hot tempered, or argumentative, mixed up a few pronouns - passed to a subeditor - who thought the article was supposed to be about Kate and did a quick edit, published it to the online paper, the writer spots it realises the mix up and corrects it.
It's surprisingly easy to make these kind of mistakes when working under pressure - your editor gives you 20 minutes to bang out a quick rehash of an old story with a new headline to make it seem new, or have noticed that an article in another publication is doing well in the search engines and asks you to take a chunk of it to get some clicks too. You pass to the subeditor who has less than ten minutes to give to each article to get through his/her workload. You might even have a third person tasked to pop in and stick some fresh photos and captions in this old article - who haven't read the article properly etc.
These things are nearly always human error - people working fast and banging out pieces to meet quotas etc.