It never happened to me and would like that to happen. I just hope that no one would ask me to give it back. But if ever they message me then I will just sent it back to them since it is not really mine. But that would be an amazing thing to happen but that would be so rare.
I do not think that someone will message you to sent it back to them. It is totally impossible as he only knows the bitcoin address not your email address. But in case that it is your friend or someone you have transacted before then it could be possible.
We all wanted to happen that to us with a good amount of bitcoin. If that happens then it must be pure luck.
Yeah it would be great to receive a amount of bitcoins for free without doing anything, so far I haven't received a single penny in charity and even in future I don't think its possible.