Well, I mentioned in another thread that the Wilding Hermit often summons a Wilding only to have nothing appear on the battlefield. The Dark Sorcerer (appearing on the map due to a random event) can summon demonic allies. However, in a recent battle he repeatedly cast Demonic Ally only to have nothing appear in response to the cast. The first time he cast it, a demon appeared. The next two times nothing appeared after he summoned. This appears to be a bug?
