 The Gemara answered that Rabbi Hanina was in a different category, owing to his abundant merit (and therefore Heaven protected him).Chapter 2 of tractate Makkot in the Mishnah, Tosefta, Jerusalem Talmud, and Babylonian Talmud interpreted the laws of the cities of refuge in and 

The Mishnah taught that those who killed in error went into banishment.One would go into banishment if, for example, while one was pushing a roller on a roof, the roller slipped over, fell, and killed someone.