I don't know what I would do to be honest. I mean who knows if the guy that just sent you this is a criminal or a good man? Very tough decision. For someone like me that has a small BTC wallet and is trying to make as much BTC as possible before its too late due price going too high, it would be hard to return it, but I would feel bad to keep it too.
It would be pretty hard for somebody to inadvertently send you a bitcoin.
Yes, it's pretty hard to mistakingly change a letter for another letter and that address actually existing, but stranger things have happened. In this forum people has reported recieving Bitcoin out of nowhere, so I guess out of millions of transactions it can happen.