solar vale
Did you really mean Acquisition and not Mesmerism (steal a follower, which you can't use if yolu or the only other player is the Ninja, because that can't have any followers)? Regardless, there are other cases where you might end up with a spell that you can't use, which is frustrating, but not a bug. There are, however, ways to 'lose' spells, though I think most of them are in various expansions (e.g., one flavour of Dragon Rage will cause you to discard a spell, or you can sell spells in the Arcane Archive, and there's the Nullify spell, which zaps all of a character's spells).
That's not to say there isn't a bug somewhere else, but I just wanted to note that it's possible to be stuck with an unusable spell when everything's working correctly.
river wing