Receiving ETH, BTC or any other altcoin by a mistake is pretty impossible because there are so many possible crypto addresses (billions of billions?). So he has to know your address and then send it to your wallet for example by copying wrong target address. And what I would do with it? Nothing, only if he contacts me for example by sending me a token with the name "Sendback" for example, I will return. 
But you never know why you receive the transaction and what his intentions are. 
This is true, on what basis is the transaction being done needs to analysed. What's been mentioned is the true fact on sending back funds. The chance of sending funds to any of the ethereum address by mistake is very rare. In my opinion the chances were low as 1%. If I'm wrong, please correct me. Every ethereum address won't vary with one or two letters, each address varies entirely. But you never know why you receive the transaction and what his intentions are.