My answer to What could possibly make magic or spells fail to work for someone?
Answer by Alex Sumner:
(A2A) A lot of fluffy-bunny new-agey type magic folk are not going to like my answer, but what the hell, here it goes:
The primary cause of spell-failure is because the magician has tacked a “an it harm none” clause onto the end of his or her spell-intention. Then, when the spirits who have been charged with causing the spell-result to manifest calculate that the only possible way of success is in fact by hurting someone, they are forced to give it up as a bad job.
The idea of a “an it harm none” clause comes mainly from Wicca, which I believe was a ploy by Gerald Gardner to make his brand of witchcraft seem respectable, although I have seen similar practices in other magical traditions. In each of them, including Wicca, the idea of adding the “an it harm none” clause is taught quite dogmatically.
In my experience, far more success tends to happen if instead of specifying “harm none,” one says “and no *innocent* person shall be harmed.” This has the added bonus of creating some hilarious scenes, although obviously not all the people involved will find it amusing. 😉