Then the hero was caught in a spell, which ended with his facing the warlock. And I knew I had to introduce another character here. And spontaneously, it occurred to me to make this particular warlock a specialist in a different kind of magic than the one that caught my hero -- this makes introducing the other character easier -- and he was given the spell by another. Even as I had him snark about its not being a very good spell, I was pondering how to limit it.
Because effectively, it means that any warlock can produce a spell at any time. No matter how obvious it is from mists and labyrinths and illusion that this warlock put his study time into deception, he could still set you on fire with a fireball. Perhaps he learned it by rote, but you would still be on fire.
On one hand, it does make the characters' fears of being trampled by them reasonable. On the other hand, I do have to keep readers from wondering why it hasn't happened already. . . .