1) Real fighting is not cool-looking like it is in the movies. Even if you train cool-looking techniques, fighting still doesn't look like that.

2) Never underestimate size.

3) Never underestimate strength. This one and the previous one apply especially to skinny people who have only sparred with other skinny people in their class.

4) There's a good chance that you won't be able to react faster than your opponent can act. See also #5.

5) The person who starts the fight is often the person who ends the fight.

6) Fighting is scary. Your hands might shake. Your knees might shake. You have no control over this. Your fancy techniques requiring fine motor control are useless.

7) Real ninjas didn't use straight swords. Those were invented by Hollywood