Regardless of the mechanism for misleading the oracle, its predictions for the future ought to become less accurate in proportion to how useful they have been in the past.

"What will the world look like when our source of super-accurate predictions suddenly disappears" is not usually the question we'd really want to ask. Suppose people normally make business decisions informed by oracle predictions: how would the stock market react to the announcement that companies and traders everywhere had been metaphorically lobotomized?

We might not even need to program in "imminent nuclear threat" manually. "What will our enemies do when our military defenses are suddenly in chaos due to a vanished oracle?"