To amplify just a little on what GaBandit said, when you find that your corner has you leaned over to your pegs AND hung off the bike... you are using the full envelope... there is nothing left for the UNPLANNED. On the track this is fine because there isn't supposed to be anything popping up UNPLANNED on the track.
On the street you are playing Russian Roulette because you have no control at all of that environment. If you are at 10/10 there is nothng left. If bad luck taps you on the shoulder then, you are down.
Bad Luck: Patch of dirt, gravel or oil. A cat, dog, cow deer, stupid cager, piece of trash in the wrong place. Pot holes. Camber change. Bug in your helmet.
And if you go down, your luck is just beginning. Think of all the nice things to roll up against: trees, cars, curbs, signposts, fences, barb wire, armco barriers...