Here we have Mishnah 2 in the second chapter of Maseches Makkos. The Mishnah rules that one who throws a stone into a public domain and kills a person is exiled.
The Gemara narrows the case:
The Gemara explains that the ruling cannot be taken at face value, for one who simply throws a stone into a public domain has committed an act that borders on the intentional, and because of such extreme negligence exile would not be a sufficient punishment. The Gemara therefore establishes the Mishnah in a precisely defined case:
The place: an area in the public domain that people use for relieving themselves, but which they are accustomed to use only at night and not during the day.
The time: the thrower cast the stone by day, at an hour when people are not normally found there.
It is this combination that turns the act into a genuine accident, a killing that is truly unintentional.
The statement of Rabbi Eliezer ben Yaakov:
Rabbi Eliezer ben Yaakov adds a point regarding the throwing of the stone: if, after the stone had already left the thrower's hand, a person stuck his head out of a window and took the blow, and died as a result, the thrower is exempt from exile, and this is not considered a genuine accident. For the Torah says "u'matza es re'eihu va'meis" - "and he finds his fellow and he dies" - excluding a case where the victim brought himself to that spot. The one who was killed was not in the place at the moment of the throw, and it is a requirement that he be there at the time the stone was thrown.
One who throws into his own courtyard:
One who throws a stone into his own courtyard, which belongs to him, and kills a person, is judged according to whether the person who entered had permission to be there:
He had permission to enter: the thrower is exiled, since he should have anticipated that a person might be there.
He did not have permission: the thrower is not exiled.
The basis for this distinction lies in the wording of the verse, which speaks of one "asher yavo es re'eihu ba'ya'ar" - "who comes with his fellow into the forest" - and is killed there. From the fact that the Torah chose the example of a forest we learn: just as a forest is a place that both of them - the one who causes the damage and the victim - have permission to enter, the verse comes to exclude the private courtyard of an individual, where the thrower is the owner and the victim has no permission to be, and such a case is therefore excluded from the law of exile.
An additional explanation: "lachtov eitzim":
Some explain the verse from its conclusion - one who enters the forest "lachtov eitzim" - to chop wood: just as chopping wood is an optional activity, the verse comes to exclude acts that are a mitzvah, such as:
A father who strikes his son as part of raising and educating him.
A teacher who strikes his student as part of educating him, in the eras when corporal punishment was the accepted practice.
An agent of the court who comes to administer the punishment of lashes.
If a person was accidentally killed in the course of any of these actions, it is a terrible thing, and nevertheless it does not carry the obligation of exile, since exile applies only to one who was engaged in an optional activity.
In closing, it is worth recalling something that Rabbi Yaakov Weinberg used to say: although in the time of the Sages, and for generations afterward, corporal punishment served a purpose and was effective, in our times - since society is what it is and expectations are what they are - it does nothing but damage. There is no benefit in it, and once it has become harmful it is no longer permitted.