Lessons learned?
1. Have a gun.
2. Have a gun that you can shoot accurately, at a distance. I'm guilty of sometimes toting my j-frame, not exactly a long range gun, and feeling that it is sufficient, since most engagements occur at close distance. Well, this one didn't. I'm not sure of the exact ranges involved, but I'm pretty sure they were outside of my comfortable engagement distance with my Model 640.
3. Have enough ammo. See my comments on the j-frame above. 5-for-sure is great and all, but with one reload you have a total of 10 rounds, with a pretty decent pause between #5 and #6.
4. Have a cell phone. Use it. Read Jeff's posts again.
5. Be able to display your credentials quickly (LEO specific). As a cop, I carry a badge. So did Officer Hammond. However, Hammond said that his badge was in his wallet, and he could not reach it easily in order to display it to the first uniforms to show up. I usually carry mine on a belt clip, beside my IWB holster, but I sometimes carry it in a badge wallet, strong-side. I plan on never carrying it there again.
6. Have in mind what you will do and say when confronted by a uniformed LEO. This sounds stupid, but when the S hits the F, simple things become hard. If you have in mind that you will lower your gun to low ready and say "DON'T SHOOT! OFFICER, I AM AN OFF-DUTY-OFFICER/CCW-PERMIT-HOLDER! THE SHOOTER IS DOWN THERE! WHAT DO YOU WANT ME TO DO?" you will be much less likely to do something that could get you shot.
7. If the nice man in the uniform tells you to do something, do exactly what he says. He's having a bad day, too.
8. Realize that you're just one lone guy playing in the ultimate team sport. You don't have to kill the guy to make a difference. You don't even have to engage. But if you do engage, remember that just exchanging shots with him can pin him down for long enough for the heavy hitters to show up.
9. Backstop. Notice that Hammond was up high, shooting down at the BG. While he could still get ricochets off of the floor, he was in a much better position than if he was shooting at a target at the same level. The reason I say this is that we imagine that once the shooting starts, everyone will leave. They didn't! Hammond himself wandered into the gunfight and didn't realize it until he saw bleeding people. There will be people around, either down the hallway, or behind walls that are concealment but not cover. There's really no way to make sure that your shots are 100% safe in a shopping mall, but you should at least be aware of the potential dangers of missing and do what you can to minimize the risk.
Mike