I don't think that's the case everywhere. In Switzerland at least, if I send you money by mistake, I can claim that money back and you would have to give it back to me, even if you have spent it already - unless you can reasonably claim that you were unaware that this money was not meant for you.

As a result, if I received a large amount of money that I coulnd't explain, I would contact the sender or the bank first. I can't just spend it immeditaly and claim that I had no idea.