I think if you explained this in layman's terms to "a bloke down the pub", he'd consider it theft. The law generally feels the same way. (IANAL.)
Not the same, but it reminded me of years ago, I had £50 deposited in to my account, by mistake, a couple of months running. I asked my girlfriend's dad - who actually was AL (WAL?) - if I could keep it. He said it would be like someone parking their car on my driveway and leaving the keys in the ignition. Inconvenient, yeah, and I don't want it there - but it doesn't make the car mine.
Depends on a country. In Poland, if a company sends you money, once you spend it, they have no right to get it back. The reasoning being that you have a right to assume companies know what they are doing.
If a private person sends you money, you have to give it back.
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.
Not the same, but it reminded me of years ago, I had £50 deposited in to my account, by mistake, a couple of months running. I asked my girlfriend's dad - who actually was AL (WAL?) - if I could keep it. He said it would be like someone parking their car on my driveway and leaving the keys in the ignition. Inconvenient, yeah, and I don't want it there - but it doesn't make the car mine.