Got to thinking about this while riding a bus in another town.
Suppose the you are on a city bus some time when it's crowded. You are sitting toward the back. A bunch of delinquent teen girls spot an enemy on the bus a couple of seats ahead oh you and close in. They are starting to get violent, clearly they are going to escalate to bodily harm, and do so, but the driver has not done anything to indicate he is aware or doing something about it. A knife comes out.
You are carrying concealed. Carrying may or may not be illegal on the bus but you have not checked (good luck tracking all such laws/regs/rules). There is no one between you and the closest of the gang, and any pass throughs would just hit another of the gang, and the angle is upward through the roof anyhow (velocity loss might make the bullet essentially harmless, if it hit someone outside the bus. More likely it would hit a car roof.)
Intervene? When and how?