Imagine opening the banking app on your phone and looking at the balance. It seems surprisingly high. You look more closely, and don’t recognise the recent transactions. Then you see the name: it is not your account. When you’re sorting out whether you feel the thrill of eavesdropping or the fear that you’ll get in trouble for somehow hacking into a stranger’s account, you might shortly feel the dawning horror that someone else could be looking at your own account right now.