Some years ago, I was having a fantastic evening at a karate class. The stone floor of our dojo was pounding with 12 karatekas practicing their kicks.
Jump. Turn. Kick. Land. Repeat.
The magic of a black belt class is that there need be no instructor to get people to practice their asses off with face-breaking concentration.
We all thought we were very cool. We'd all mastered a kick that had taken us years to come to grips with. The punching bag in the middle of the room didn't stand a chance. In a silent line, turn by turn, we faced it and bowed. And then, jumpturnkicklandrepeat. For good measure, we'd throw a couple of dead-straight punches at the thing as soon as our toes touched the floor.
I did it once.
I fell. Great.
I did it again. The sweet thud of foot slamming onto the side of the bag, and then the tremor on the floor as I fell back to Earth. Success! And again. Harder. Harder! And for good measure, punchpunchslam! My whole body was on fire and my mind was a crystal clear sea, cool as ice.
I thought: 'I didn't know I could think, two feet off the ground - but I can! I can see exactly what to do! If that bag were a person, he'd have a shattered face and he wouldn't have a chance.'

The dojo is not just a room - it is a sensient being. And it detests boasting about face-breaking prowess. It hates pride. It sensed mine rising up from my insides into my smile, and stepped in.
The sensei behind me tapped my shoulder and said, 'Good!! Excellent. You got it. Now do it with your eyes closed from now till the end of the class.' One and half hours.
Fuck.
I was too far gone with my euphoria to watch my tongue and said, 'That's impossible. Even you can't do that.'
Mistake.
He did - Eyes closed, two kicks before landing, one turn. Punchpunchslam! Eyes open. For a second I thought I imagined it. But the bag was swinging wildly from the ceiling and everyone's jaws had hit the floor.

I tried it and of course, my forehead connected with the bag. Hard.
I tried it again. No luck.
Everyone who'd mastered the kick tried it. We all fell.
It's a pretty naked feeling, to wear a black belt and fall with your eyes closed onto a cold stone floor, panting, in front of people you respect and who've broken your bones as you fought your exam. We all felt about 6 inches tall.

The second you think you know - you don't: I've never forgotten that.