Masechet Bava Kamma, Perek Gimel, Mishnah Vav. Mishnah says that if you have two people moving in a public area, one of them was running and one of them was walking. Normally, we would think that’s a problem. If one of them is walking in the Reshut HaRabim (public area) and the other one is running—what she shouldn’t be doing—then the one who is running is causing the damage. But the Bartenura points out, as the Gemara does, that this is a case of Chasorei Mechasra Vehachi Katani: we have to add into this Mishnah more context.
The context is that we’re talking about a situation of Erev Shabbat (Shabbat eve) or Erev Yom Tov (holiday eve), where a person is allowed to run in the Reshut HaRabim. It’s considered a time when a person is rushing to do Devar Mitzvah. He’s going to do something of importance for Shabbat or Yom Tov (holiday) in order to prepare the needs of the holiday or of Shabbat. And therefore, even though he’s running, we’re going to point out that he is patur (exempt from payment), which is just pointing out that he’s running in an expected way. He should be running on Erev Shabbat. It’s understood why he’s running on Erev Shabbat. In the next case, on a regular day, both of them are running. We say they accidentally hurt each other. They injure each other. So the forces of both bodies in this second case are causing the injury. In both of these situations, we say that they’re equally responsible and they’re both exempt from payment.
In the first case, when the person was running in the Reshut HaRabim, he was doing it in an allowed way. It’s understood that people run on Erev Shabbat and on Erev Yom Tov. So when he ran into somebody else, that person should have expected that he was there. And the runner would not be responsible on Erev Shabbat. In the second case, where they were both running, they were both equally responsible for the damage that was caused, because they were both running into each other. Neither of them is more responsible for the damage than the other one.
The implications of this Mishnah, though, are that on a regular day, not on Erev Shabbat or Erev Yom Tov, if one person is running in the public area and the other one is walking in the public area, the one who is running is doing something unexpected, and therefore he would be chayav—he would be liable for causing the damage. And that’s why the context of this Mishnah matters. The only time we say the one runner is patur is if it’s Erev Shabbat or Erev Yom Tov. But if it’s a regular weekday, then the runner in the Reshut HaRabim who hits a walker would be responsible for that damage. But as we pointed out, if you have two people running, then they’re both equally responsible for the damage, and therefore both of them would be patur.