A couple times, at least.
Once in college (nearly 20 years ago!) when my roommate (another early 20's female) and I lived in an apartment in a borderline-scary part of Long Beach, CA. At 2 am my roommate came into my room saying an unknown male was knocking on our door. I could hear the knocking and a male voice, but I couldn't make out what he was saying. We called 911 and the operator told me that officers would stop by if they got around to it, but it would be at least an hour. Very reassuring. Eventually the guy stopped knocking and left (I did find out, a few weeks later, who the guy was...but that's a whole other story!)
A few years ago, we were driving on the freeway and 2 cars started driving like crazy--boxing us in, tailgating us, cutting in front of us then slamming on breaks. The two cars were working together to terrorize us--no idea why. It was terrifying though, really. I was the passenger and called 911. This went on for about 10 minutes, then the cars sped off. We never did see any highway patrol/police respond.