We have all read stories of coins being sent to wrong addresses (typos or badly copy and pastes)
What would you do if 100 Btc landed into your account? (unfortunately this has not happened to me yet)
I will be surprise at first but I cannot spend money that wasn't really mine. I will return it to the owner less the transaction fees. If the owner will reward me some BTC because I returned his', I will gladly accept it. I will look for a way to communicate to him and will tell him to be more careful next time.