Cassandra, the daughter of Priam and Hecuba, and the sister of Hector and Paris/Alexander, made a number of predictions before the Trojan War and during the course of the war. Therefore, answering this question with any precision may be difficult.
However, this question may recall a prediction supposedly made by Cassandra when Paris/Alexander was discovered as Priam's long, lost son. Cassandra apparently warned her father than Paris would cause the ruin of Troy if Priam took his son back. Priam ignored the prophecy, as everyone did with Cassandra's prophecies. We should note, however, that this prediction does not occur in Homer's Iliad.
The second book of Vergil's Aeneid also contains an appearance by Cassandra as the Trojans were trying to bring the wooden horse into the city. Cassandra warned the Trojans against bringing in the horse, but this prediction was also ignored.
If the Trojans had been able to heed either of these prophecies, then the fall of Troy would have been avoided.