This seems very surprising to me. And it also seems criminally exploitable.
So if a bank in Poland sends you a million dollars by mistake, is it yours?
What would happen if someone stole it and transferred it to your account, does that make you a criminal?
And even more interestingly, if a malicious bank programmer (who's in league with you) introduced a bug on purpose (let's suppose we cannot prove the intent here, it's very difficult) and the bank mistakenly deposits you the money, do you have a duty to return it?
I don't know about Poland, but in Austria there is a similar law (IANAL but my lawyer friend told me about it), and banks are excepted from it. So if a company sends you money by mistake you can spend it and don't have to give it back after that, but if a bank makes a mistake in your favour, you have to return it in any case. Happened to my lawyer friend's mother.
So if a bank in Poland sends you a million dollars by mistake, is it yours?
What would happen if someone stole it and transferred it to your account, does that make you a criminal?
And even more interestingly, if a malicious bank programmer (who's in league with you) introduced a bug on purpose (let's suppose we cannot prove the intent here, it's very difficult) and the bank mistakenly deposits you the money, do you have a duty to return it?