The scenario you describe happened once - at Lexington, MA on April 19th, 1775.
The forces sent to disarm the citizens were at a bit of a standoff with the fellows standing in the middle of town. The citizens were actually leaving.....and a shot was fired - without the command to fire. What ensued was a massacre of Lexington citizens, followed by a rout of the occupying forces.
The shot was that was fired almost certainly came from the British, but I doubt that it would make any difference in the days events had it come from the crowd.
Should an event like Ferguson get a nudge like that, I have no doubt, the situation would escalate muy pronto. I do doubt that the citizenry would unite over that particular cause, more than likely it would be limited to that area. For a while.
I used that analogy over in the GD thread. IMO Americans are ripe for something like this, not that I look fwd to it but I think its a real enough possibility to keep an eye on. Esp if there is a trial and it proves justified. ..rightly or wrongly.