One of my champion had the ability to spend less mana when casting spells in tactical combat.
And when I'm short on mana, and I should be able to cast that damn last fireball to save my life, the game won't let because it just look at the real cost of the spell. The mana spent is always what it should (the reduced "price").
For instance a spell that cost 5 only cost 4 with the perk. I have 4 mana. I can't cast the spell. If I have 5 mana, I can't cast it and it usses up only the 4 mana.
The game shouldn't look at the base cost of spells when deciding if I can cast it. It should look at the cost for the selected champion.