When you see someone doing something badly, maybe it's his first try. The first time I try something, it takes me longer than it should, and the result is not very good.I have seen many cases of folk, bashing the living daylights out of a tree in the name of using an axe, and striking ferro rods with knife blades and spluttering through smoke while lighting a fire and many more things, all because they try to short-cut the intricate nuances that come with proper experience.
If somebody looking over my shoulder says "you're **** at that", it doesn't exactly encourage me to try again and get any better.
And saying "you're doing it wrong" doesn't help much either.
Much better is to ask "why do think that didn't work?" Encourage the student to think through why it didn't work, analyse the practise compared to the theory, and discover for himself what went wrong. This is what encourages proper self-evaluation and improvement.