cellio | daf bit: Bava Kama 32

The mishna teaches: if two people are passing each other on the street, either one walking and the other running or both running, and they collide and someone is injured, neither one is liable. Why are both cases stated? If there is no liability when one is walking, then surely there would be no liability if both are running! And how can there be no liability when just one is running -- he's doing something unusual so it's on him, right? The g'mara concludes that the case of one walker and one runner occurs on erev Shabbat before sunset, when running is acceptable behavior, while if they collide on a weekday the runner is liable to the walker. Why is running on erev Shabbat permissible? L'cha dodi likrat kallah -- come let us go and greet the [Shabbat] bride. Rabbi Yannai used to instead stand and say "come thou, oh queen". (32a-b)

This mishna follows one about collisions when one person is walking in front of another, e.g. if the log I'm carrying breaks the pot you're carrying. The rules there are what I expected: if you hit from behind you're liable, unless the guy in front stopped suddenly, unless he warned you he was doing so.