We had a case here where police burst into a home on a no-knock warrent, the home owner shot a police officer while his daughter called 911 for help. the police charged him with murder, but the Judge decided it was justified self defense and that charge was dropped. It's very common for bad guys to yell police up here while raiding a growop, as the law up here is so lax, the average bad guy does not even run and knows they will just get probation again. So by yelling "Police" there is less chance of them defending the growop till it's to late.