Regarding prophecy, the Zondervan Encyclopedia states:
Sometimes one feature of a prediction may involve a condition though the prediction as a whole is unconditional.
The conditional nature of a prophecy is not necessarily explicit. That is, there may be no "unless" or "if" in the original prophecy, no indication that the prophecy is avoidable or could potentially be delayed or altered.
A clear example of this is Elijah's prophecy regarding King Ahab.
1 Kings 21:19 reads:
Say to him, ‘This is what the LORD says: “Haven’t you committed murder and taken possession of the property of the deceased?”’ Then say to him, ‘This is what the LORD says: “In the spot where dogs licked up Naboth’s blood they will also lick up your blood – yes, yours!”’” (1 Kings 21:19 NET)
However, Ahab humbles himself, and thus it is proclaimed that the judgment will not fall on Ahab himself, but on his house later on. The prophecy is fulfilled when Jehu has the body of Ahab's second son thrown into Naboth's field (2 Kings 9:26). Thus the prophecy is fulfilled, although not on Ahab himself as originally predicted, but in its altered form.
I want to consider a second possible example: the destruction of Nineveh. Jonah prophesied that the city would be destroyed in 40 days, but God relented in view of the city's repentance. However, Nineveh was eventually destroyed as prophesied by Nahum.
Can Nineveh's destruction be viewed as a fulfillment of Jonah's prophecy, delayed or altered in some fashion by the response of the Ninevites? Or should the eventual destruction of Nineveh be seen as a fulfillment of Nahum's prophecy alone, and completely unrelated to Jonah's prophecy?