I'm running a DK fire mage/healer and I've found that some abilities like fiery claws can be used even when the target is too far away, just nothing happens. While I understand that the spell in my example is an AoE spell as far as I know it only targets the mobs in your range of vision. So when I have my target cursor on the single NPC I can see and it's too far away I can still trigger it. I wish with spells like FC that it would either grey out if it won't land on the target, or give you an indication (like most other AoE ones do) where the spell will land so you can move closer if needed.