Fervent Denial can't be cast from the graveyard during damage prevention or regeneration (to counter a Healing Salve or Death Ward, for example). Neither can any other spell, nor other abilities activated from the graveyard or exile. The only other one I've found so far is Embolden, which I haven't implemented, but that'll change once effects that let you cast arbitrary spells from the graveyard/exile like Recoup or Psychic Theft go in.