Only once, but it was a good chance to try out a technique I'd learned from Derren Brown (British street magician, NLP practitioner, Psychologist, etc), he was drunk and thought his girlfriend had pointed me out (no idea what for), he came storming over to me shouting over to me to start something. I turned around, big grin on face (shitting myself inside), and pretended he was an old school friend I hadn't seen since I was about 7. I rambled about how good it was to see him, had a laugh with him about my old teacher, and reminisced over a song the school had everyone sing. I cheerfully laughed it over with him, pretending he knew exactly what I was talking about.
Of course he had never met me, but he was drunk, and I was keeping the conversation moving fast enough for him to not maintain his bearings on it. The effect is known as ( I think) an adrenaline dump. He can't be that confused and off guard while being aggressive, so all the momentum and aggression are replaced by this intense disequalibrium. It was at that point that I said my goodbye and swiftly darted away from him. The look on his face was priceless and I was so thrilled it worked.
In that scenario, you could easily get a good first hit off on somebody if a fight's unavoidable (or in Derren's case hypnotise them), I didn't because I fight like a octopus having a seizure. You have to think fast, or (like I did)have a speech planned out for if the situation arises. Though I think I was just lucky he was drunk.
Plus: I'll look for a link to that 'adrenaline dump'.
Edit: Okay, after some light research, I can conclude that the 'adrenaline dump' is actually the rush of the adrenaline preparing for fight or flight, which you then need to dissipate and ultimately reverse in the guy threatening you.