The first time that my aikido really surprised me with its effectiveness was during my 4th kyu testing. I was working with two uke, and I blended perfectly with two shomen uchi attacks, resulting in two beautifully executed techniques. It felt like I'd hardly touched them. (
This was due, no doubt, to quite a bit of good luck and not to a high level of skill. 
) Regardless of it's cause, this occurence just plain
freaked me out. 
I looked with wonder at my sensei, as if to say "Holy crap! Did you see that?! Is that how you feel all the time?!".
While I stood there slack jawed, one of my uke popped up and nailed me on the noggin. So I achieved a personal victory that night, but I still had the outward appearance of a colossall screw-up.
