Be warned: spoilers ahead...

When playing against Yog Sothoth and the Arcane Understanding mystery comes into play you must cast a spell, resolve it's effects, spend a clue and discard that spell to place an Eldritch token on the mystery card. Now, technically, I know you can throw a spell away. For instance, cast Mists of Releh even though there are no monsters on your space, resolve it, spend the clue.

However, that feels like a cheat to me. When solving this mystery I will only allow myself to cast a spell if it's effect is valid. In the above example I will deliberately put an investigator in harm's way and move it to a space containing monsters just to be able to cast the spell. This makes the mystery more challenging but it feels more correct to me.

So I'm curious. How do you play it? Will you cast a useless spell just to solve the mystery, or do you prefer to, like me, take the hard route and drag this particular mystery out forever?

I generally play it that if the effect isn't a valid effect, you can't use it. So in the Mists example I couldn't cast it unless there really were monsters to evade. Likewise I couldn't choose to fail a combat roll so I could block the damage with Inspire Bravery/Flesh Ward, there must be Clue tokens in play to use Clairvoyance, etcetera. I do consider it an option to cast the spell when it would normally not be strategically sound to do so, but otherwise, you need to fulfill the conditions of the spell.

I agree with the two of you. My general rule when playing is to go with an interpretation of the rules that makes the game more difficult to win! hehe.