Funnily A scenario like this actually happened to me, A group of youths rampaging through a park late at night surrounded my brother and I, but through the gloom and their own drunkeness they saw that we weren't easy pickings, we both are props that play club rugby. They soon ran off and hid behind a fountain, out of the shadows 2 constables with batons drawn, then proceeded to confront me and my brother and asked us where the group of teenagers had gone, I asked what they had done and the constable explained that they had just beaten and robbed a young couple out for an evening stroll. I immediately turned around and fingered the group behind the fountain and proceeded to enjoy watching the police tear into them lol.
Some years later I was walking alone from my local pub in the same area, and a guy with no warning ran up on me from 20 meters out and attempted to plant an elbow on my nose, I dodged this so that he caught me a glancing blow, When I turned to confront my assailant low and behold it was one of the youths that had been arrested for the mugging, seems some people just don't learn their lesson on what polite society is all about and he obviously blamed me for arrest rather than his own actions.
I then proceeded to kick his ass, at which point when I had him restrained in a head lock and was driving his face into a pole (much to my shame) his dog mates then jumped me from behind, as people of this moral ilk are want to do they were in a pack. Unfortunately for them this was right out the front of my local pub of whose patrons I was on first a first name basis with. Most of whom had witnessed the initial assault and had enjoyed watching me hand out some street justice. They had started to pile out of the pub when they soon saw what was going on. Ironically it was me who prevented it from further escalating by telling everyone to go back inside and telling the dick wits to piss off, something I'm sure these fools would never do if the shoe was on the other foot.