story
I've often had the pain of dealing with people who try to do it all in their head. Sure, one or two are good enough where it's not a problem. The rest of them? They keep getting confused and making mistakes. For things that are not very complex. If they had only written it out or drawn on paper/board, they would not get confused.
It's most obvious when I try explaining some code's algorithm to them. They'll begin with understanding, and then 70% of the way through they'll keep getting confused because they couldn't hold it all in their head.
I've often had to tell these people "Don't try to do it in your head!"