Knowledge pool requires a game action to cast the spell you select from the pool. You're allowed to select the spell, but you can't actually cast it until you tap and untap a land, for instance.
This is true, plus...
1. For controller, Knowledge Pool apparently works properly. But NOT for opponents.
2. Land tap allows opponents to cast, but Land untap may drive another problem, i.e. untapping causes opponents to waive the timing to cast spell. Of course nothing is cast, simply loss.