We are all human, including the banks, even though some of us believe that banks behave in an in humane way.  This means that banks make mistakes too, although I doubt many of them would be prepared to admit it.
Usually the mistake is to the customer's detriment.  I have had far too much experience of fighting with banks over misuse of their ts and cs, such as withdrawing overdraft facilities in bad faith, or failing to honour previously agreed criteria.  I must say that these days they are much more commercial, but things do still go wrong.
So what happens when the bank's mistake is your favour?  Are you under an obligation to tell them, and if they notice, are they entitled to help themselves to whatever compensation they feel they are entitled to out of your account?
Well as always, it depends on the circumstances, but whilst banks can sometimes act like they make the rules, they are still governed by the same laws as everyone else are.  In fact, in some respects they are in a worse position because not only can you sue them, but you can also report them to the banking ombudsman or the Financial Conduct Authority.  The banks are not as powerful as they might sometimes like you to believe and you do have alternatives.
If you're in dispute with a financial organisation and you want to know whether they've gone overdrawn on their obligations to you, drop me a line at stephanie@kleymansolicitors.com.  We can meet for coffee and a chat about who is in the red.
Kleyman & Co Solicitors.  The full service law firm.  You can bank on us.