I believe my first conscious experience of this was when I tried to write a towers-of-hanoi program. http://en.wikipedia.org/wiki/Tower_of_Hanoi

My 'hands' figured out the algorithm in a minute or two, and could fly through the stacks without a problem. But when I tried to be mindful enough of what I was doing to write an algorithm for it, I basically forgot what i was doing, and had to painstakingly reteach myself.