My first instinct would be to take out my cell phone (which I ALWAYS have with me) and call 911. Either that, or I'd run like h*ll and go look for help.

A few weeks back I was working and heard some shouting. A coworker who was not allowed to move from her post called me over and said someone was stealing a woman's purse. I yelled for someone else to help, but by the time I put together that the thief was running for the door and that noone would stop him, it was too late. The person got away, because I hesitated.

It's very likely a crime will be committed right in front of you and you may not be able to recognize it. This is a shame, because we are the second best defense, besides the police, to keeping crime at bay.

There are multiple instances of crowd mentality, in which the people have watched, recognized, done nothing, and in some cases even participated. The recent rape and assault of a 15 year old girl in California has me doubting precisely what people would do in that situation, although that was a pretty isolated incident.

A news event that happened many years ago that became popular in fiction (though the event really happened) was the stabbing of Kitty Genovese, she was coming home and stabbed twice, screamed, people came out and watched, uncertain of what to do.
This is known as the Bystander Effect.

The last problem with helping in the event of a crime is if you're currently at work. Jim Nicholson, a Seattle bank teller, had his instincts respond when someone came in and asked for money (robbery). Jim chased down the man and caught the criminal, only to be fired due to bank policies. It's understandable, it's a dangerous thing he did.

I like to believe that I'd help stop the crime from being committed, but I'm concerned one of these 3 things may stop me.