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. 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.