Every once in awhile, you hear someone say that XYZ was a loon (fill in the XYZ with your favorite lunatic), therefore they were wrong about some magical or spiritual technique. Personally, I find this to be the biggest cheat ever to prove that someone was wrong. As one of my friends says, "Even an idiot (or lunatic) is allowed to have a good idea occasionally---the trick is to spot and steal it as quickly as possible."
Let me tell you a secret. We are all loons. It is only a matter of how loony we are. Name your favorite occultist...they are a loon. Name your least favorite...they are a loon. In fact, everyone in the world is a loon.
Here is another secret: In the midst of their maddness, every lunatic's world has a logic to it. So when you hear that so-and-so was a loon, and therefore some technique is wrong, you can either wholeheartly accept that the technique is wrong or you can attempt to understand the logic behind the technique.
Often, it turns out that a loon will have completely loony ideas. But every once in awhile, they stumbled across a good idea. And you would have to be a lunatic not to steal a good idea that is not bolted down.