From this article in The Nation:
A case in Texas two weeks ago highlights the risks of civilians intervening in chaotic situations. Police say that as two carjackers struggled with the owner of a car at a gas station in northeast Houston, a witness decided to take action into his own hands. He fired several shots, but missed the perpetrators and shot the owner of the car in the head. He then picked up his shell casings and fled the scene. Police are still looking for the shooter.
The potential for that kind of outcome is why most police agencies strongly recommend that concealed carry holders only use their weapons as an absolute last resort, and not intervene in robberies or other crimes in which they’re not directly involved. David Chipman notes that even police officers are told that if they encounter a crime in progress while off-duty, “maybe the best thing to do at that time is not to take lethal action but instead try to be the best witness you can be.”
I carry a knife with me almost everywhere I go. I harbor no illusion that using it in a violent situation would be a Bad Idea. I think people are really deluded if they think they can spend time a little time at the range and avoid making a bad situation worse.