[gjsman-1000]

Well, there is actually an exception to that too - the Police can break into any home, without a warrant, in the US, if they can prove they had reasonable cause to believe that there was imminent and immediate threat of bodily harm or death.

If you are a police officer and see a guy clearly pointing a gun at someone else through the window, yes, you can break in if the circumstances warrant that.

[05]

Or if you receive an anonymous call from a swatter..

[JoshTriplett]

The flip side of that is that if the police enter on that basis, any evidence they come across in the course of that action is going to be tainted and potentially thrown out of court, especially if that evidence wasn't in plain sight.