The Trojan princess Cassandra was cursed by Apollo that nobody would ever believe her. Apparently though, the curse wasn't completely effective:
After Cassandra grew up, she again spent a night in the temple. This time the god appeared and tried to get her to yield to his desire, but she refused. For this affront to him he punished her by causing her prophecies, though true, to be disbelieved. This curse failed to be effective on only one occasion. When Paris appeared as an adult at the court of Priam, Cassandra declared him her brother. It had been accepted by everyone that he had died in infancy from being exposed.
Source: Women of Classical Mythology: A Biographical Dictionary, Robert E. Bell
One version of Paris' return is told by Hyginus, in his Fabulae:
 XCI. ALEXANDER PARIS
After Priam, son of Laomedon, had had many children by Hecuba, daughter of Cisseus or of Dymas, his wife, again pregnant, in a dream saw herself giving birth to a glowing firebrand from which many serpents issued. When this vision was reported to all the seers, they bade her slay whatever child she should bear to avoid its being the ruin of the country. After Hecuba gave birth to Alexander, he was handed over to be killed, but the servants out of pity exposed him. Shepherds found the child, raised him as their own, and named him Paris. When he came to young manhood, he had a favorite bull. Servants sent by Priam to bring a bull to be given as prize in funeral games in Paris’ honor, came and started to lead off the bull of Paris. He followed them and asked them where they were leading him. They stated that they were taking him to Priam . . . [to be prize] for the victor in the funeral games of Alexander. He, out of fondness for the bull, went down and won everything, even over his own brothers. In anger Deiphobus drew his sword against him, but he leaped to the altar of Zeus Herceus. When Cassandra prophetically declared he was her brother, Priam acknowledged him and received him into the palace.
What was so special about that particular instance? Why did the Trojans believe Cassandra's recognition of Paris?