i remember this. for at least a week people couldn't access their money. it was chaos. the bank lost lots of money and customers due to this botched transfer.
People were seeing random balances in their accounts, or seeing other people's accounts, according to some news reports at the time: https://www.bbc.co.uk/news/uk-43860449
Nobody actually lost any money. The "coins" were still mine (I was affected) and heavy banking regulation in the UK probably reduced the harm to effectively zero for the vast majority of retail customers.
I received compensation for the harm caused and got all of my money back.