I found a paradoxical niche interaction! I was playing unfair with the Monster Charged curse. If you use Fate's Strand spell (the one with Spell Rescue), the behaviour gets really weird by being able to save multiple heroes and net mana off of it.
But on the other hand, if using strand reduces your mana, which reduces monster dice by enough to save a hero, it'll refund 4 mana which means no hero is saved. But if no hero is saved, then it should reduce mana sufficiently to save the hero. Therefore, Logical paradox. Turns out the game just resolves this by not considering a hero saved.
Little gems like this are why I'm loving this game. Keep it up!