Originally Posted by Vlad

It depends on the type of warrant that they've got. In certain situations, police can obtain "no knock" warrants to allow them to forcibly enter a home without warning. In fact, I believe there was a very similar case a while back where cops executed a warrant like this at the wrong address, a resident there shot at them, presumably to defend herself, and was killed by the police.

... Just looked it up, and here we go:

http://en.wikipedia.org/wiki/Kathryn_Johnston