All have been the scene of large-scale active shooter scenarios that were the result of terrorist activity.
We, whether duty-bound personnel like the police, private security entities, military, "other government agency" type or, simply people who are concerned with out own personal defense and security, are all hanging around this website because we feel the need to make ourselves better combatants.
One thing about combat, it's a very unforgiving mistress and the wrong tactics leave permanent reminders (in the form of scars or grieving families) of tactical errors. By far one of the more common errors I see within the self-defense community is the use of similar tactics between a more common self-defense scenario and an active shooter incident.