I want to put a spell on my units, but it says I already did, yet I never did!
There are spells that take effect instantly (e.g. Sea storm) and those who only have an effect when the fight takes place (e.g. Desire).
If there is a spell on the units placed by either you or your opponent, that only takes place upon arrival (Desire, Heroic power, Aim of the Huntress etc.), then you cannot put any spell on the units unless you first use Purification. However, instant spells (Wisdom, Storm, Rage) do not block spells. They can be cast as long as there is no delayed effect spell on the units. It doesn't matter who places the spells.