Even armed police do this. Its happened here and the folks caught in the ‘war zone’ were on their own. The lesson is to be prepared.
The most famous example were the two bank robbers in LA a few years back. Armed to the teeth with automatic weapons and full body armor; local cops arriving on scene had their service weapons and shotguns. Some of the officers ran to a nearby gun store to “borrow” AR-15s and ammo. The robbers kept dozens of police at bay for an extended period; if I’m not mistakken, the police finally killed one of them and the other suspect killed himself.