There could be legal consequences if the amount has been transferred using a stolen card or a terrorist account. If someone intentionally sent you money through a stolen card, they will ask you to return it to some other account and keep a percentage of it as a reward. If such a thing happens, you should inform it to bank instead of returning the money yourself. The terrorist accounts are monitored by FBI and interpol to find their contacts. So, if they've sent you by mistake, then you should prepare an excuse for worst case scenario.
If it's just a mistake and the sender chooses to ignore it, then you can happily spend the money yourself. But in any case, it's better to wait a few days.