If someone deposited $10m in my account by mistake I'd do a runner myself, but all this couple have done is increase their overdraft, albeit by more than they expected, so it is still their debt and their responsibility.
I may be a bit of a wuss but I would give it back. Could you spend the rest of your life waiting for a knock on the door? And as for the fact that a bank made the mistake, two wrongs don’t make a right.
In the early 60's, when I worked in the City, I happened on an unattended Bullion Van loaded with 20 (that was my quick count) Bars of gold.
No Guards, no Cameras, not a soul about. I assessed that, although I could remove and carry one Bar, there was little chance that I could safely stash it and go about my business without someone noticing that I had taken longer than usual. So I left the site with little regret.
In those days, us young Jobbers made quite sufficient to pay for, the TR, the Chelsea Flat, Annabel's and Crockfords, so I wasn't unduly put out. Maybe I'd take the risk now as the price of Gold compares more favourably with my earnings as a Taxi Driver!