If a reader were to open The Once and Future King and not know what it was about, the character of Wart in The Sword and the Stone might be rather confusing. If that reader were to have accidentally skipped the first page in the novel, they would have no idea whatsoever who Wart is or why they should care about this wayward boy from medieval England. Only offhand, on the first page, does White give a quick sense of who Wart is: "The Wart was called the Wart because it more or less rhymed with Art, which was short for his real name." This is the only indication whatsoever, in the early part of the novel, that Wart is King Arthur. Without it, the reader might think this is just a random boy.
So it is quite jarring when, in Chapter 14, seemingly out of nowhere, White foreshadows with unbothered certainty who the Wart will grow up to be:
In other parts of Gramarye, of course, there did exist wicked and despotic masters––feudal gangsters whom it was to be King Arthur's destiny to chasten––but the evil was in the bad people who abused it, not in the feudal system.
This is the first instance of the phrase "King Arthur" in the text. This is such an explicit and undramatic bit of foreshadowing that it catches the reader off guard. It is, indeed, foreshadowing, as White uses one of the problems of an earlier point in the narrative (the moral status of feudalism) to describe a later point in the narrative (Arthur's Round Table maintaining peace in the land).
This unusual foreshadowing has particular effects. It confirms the total omniscience of the narrator, that it is entirely aware of all parts of Arthur's life at all times. This foreshadowing also suggests that the concepts of equality and justice exemplified in Arthur's Round Table were, perhaps, on his mind since his childhood.
It is, also, simply a funny subversion of expectations from White. The author made a bold decision by using one of the most famous characters in English literature, King Arthur, but referring to him with a false name for the first hundred pages of his novel. The reader might expect a grand reveal or tortuous quest to learn his real identity. Instead, White drops the name "King Arthur" in an innocuous bit of foreshadowing in the middle of Wart's narrative. This is one of the many ways that White manipulates a storybook structure to ironic effect.