I've noticed that sometimes when you're in the middle of a battle, trying to summon the clannfear or mage light won't work. It'll do the summoning animation and take a chunk of your mana, but the actual effect (clannfear / light) won't appear. Sometimes it works, when you try 4+ times, but that just eats up all of your mana.
This is a bug, yes? One of my friends told me it's how it's supposed to be, but somehow I don't think spells are supposed to drain mana without producing effects.