Wizard's First Rule - Terry Goodkind

“That, dear one, is the point of the demonstration. Reality isn’t relevant. Perception is everything. If you think it is the enemy, you can destroy it, whether true or not. The magic interprets only your perception. It won’t allow you to harm someone you think innocent, but it will destroy whoever you perceive to be the enemy, within limits. Only what you believe, and not the truth of your thoughts, is the determining factor.”

 

That sounds... very dangerous actually. And like something that'll become a painful issue at some point.