We had a kid we called, The Giant. You can guess why. Tall. Heavy. Hit like a truck. But a real good kid. He had made it to brown belt, and that's when he got really dangerous.
He was developing the skill to pull off some really dangerous moves, but hadn't yet realized the control to do so safely.
Maybe I'm off base here, but it seems to me that promoting someone to brown belt before they have learned control is not a great idea. It may have to do with my school, though; for us, brown belts are assistant instructors, and are tasked with being "human punching bags" for the lower belts. Control is HUGE for us--it is stressed from day one. Students who repeatedly hurt people, and can't be trained out of it, are kicked out of the program long before brown belt.