A friend of mine entered Canada by mistake. He did not have anything except a university student id card. So the Canadian customs officer returned him to the US. Upon return to the US, at the US checkpoint, the guy explained that he entered Canada by mistake and the Canadian customs officer sent him back. Then the US custom checked his school id. They held him for a while, made few calls to different authorities. Once they found out, that he was a student in the US school and was in the US legally, they let him go. He did have a valid visa (unexpired), but his passport was at home.