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

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.