There is also the question of knowing when to act, which is something that is not really trained in a dojo setting. I mean, it's possible that you may find someone setting up to strike you such that you sense it and see it coming -- if you are training carefully in your dojo you should be honing your observational skills of what preparing to strike causes to happen in someone's posture and attitude. But more likely it will either be a sneak attack or an altercation that you are secondary to, and then have to make the decision whether to get involved or not.
No one's perfect..... Nothing beats a sucker punch.....!!!
And here endeth the 2nd lesson.....