At least in some European countries you can sue a person who refuses to return the funds transferred by a mistake and win inevitably. I have been such a person

At least in some European countries you can sue a person who refuses to return the funds transferred by a mistake and win inevitably. I have been such a person who got money this way (from a particularly rich company which earns its money a legal but morally questionable way so I have no remorse) and has been forced to return + pay the legal expenses (which can be pretty affordable in Europe). I never meant to actually keep the funds forever but used the lucky event as a loan :-)