There are CONDITIONAL and UNCONDITIONAL prophecies...
1. Consider the story of Hezekiah: Isaiah said he would die, yet Hezekiah sought the Lord and God had mercy on him. God extended his life for 15 more years.
2. Consider the story of Ahab: Elijah prophesied against Ahab. When Ahab repented, the Lord told Elijah that he would not bring that prophecy to pass in Ahab's day, but in his son's days.
3. In the NT, we see when Paul was being shipped to Rome. They got into a ship wreck. Paul said an angel appeared to him saying "none of the lives on the ship would be lost." When some of the men wanted to escape from the ship, Paul said they would lose their if they left the ship. The men stayed on the ship and did not lose their lives.
There are a good amount of CONDITIONAL prophecies in the bible. The context of the prophecy HELPS us determine if it's conditional or not.
Here's an UNCONDITIONAL prophecy:
Isaiah 7:14 Therefore the Lord himself shall give you a sign;
Behold, a virgin shall conceive, and bear a son, and shall call his name Immanuel.
The result of this prophecy is not based on man's actions.