[atela]

They have to give back the principal, but what about the interest earned from their bank?

[salawat]

Banking errors are clear cut. It was never yours in the first place.

[mimixco]

Somewhat frighteningly, all money you have in the bank belongs entirely to the bank and not to you. The most they owe you is a promise to pay, but by making a deposit you give them ownership of the money.

So, if money you deposit isn't yours, certainly money you didn't deposit isn't yours.