The problem of thinking about what you are doing.

A centipede, with it numerous legs was makings it way merrily along until a frog stopped and asked

"Pray tell, all those legs, so tell me how do you know which leg to put out first?"

 The centipede had never thought of intricate mechanics of its many legs...he just let his brain, use them.

And he knew he should hurry on, but he was so distracted in thinking about how he used his many legs

that he was immobilised.

And the centipede not knowing how to run, the frog moved in for his fun.

Moral.... you are not consciously aware of  your brains ongoing activities, nor should you want to be...for like the centipede it can cause one to stall, and interfere with the brain well oiled
processes