My Mistake!

MUCH have I travell'd in the realms of gold,
And many goodly states and kingdoms seen;
Round many western islands have I been
Which bards in fealty to Apollo hold.
Oft of one wide expanse had I been told
That deep-brow'd Homer ruled as his demesne:
Yet did I never breathe its pure serene
Till I heard Chapman speak out loud and bold:
Then felt I like some watcher of the skies
When a new planet swims into his ken;
Or like stout Cortez*, when with eagle eyes
He stared at the Pacific—and all his men
Look'd at each other with a wild surmise—
Silent, upon a peak in Darien.

- John Keats, "On first looking into Chapman's Homer"

*In point of historical fact, it was the members of Vasco Núñez de Balboa's expedition who were the first Europeans to see the east coast of the Pacific, but Keats chose to focus on Hernán Cortés; "Darien" refers to the Darién province of Panama. Keats had been reading William Robertson's History of America and apparently conflated two scenes there described: Balboa's finding of the Pacific and Cortés's first view of the Valley of Mexico. The Balboa passage: "At length the Indians assured them, that from the top of the next mountain they should discover the ocean which was the object of their wishes. When, with infinite toil, they had climbed up the greater part of the steep ascent, Balboa commanded his men to halt, and advanced alone to the summit, that he might be the first who should enjoy a spectacle which he had so long desired. As soon as he beheld the South Sea stretching in endless prospect below him, he fell on his knees, and lifting up his hands to Heaven, returned thanks to God, who had conducted him to a discovery so beneficial to his country, and so honourable to himself. His followers, observing his transports of joy, rushed forward to join in his wonder, exultation, and gratitude" (Vol. III).

John Keats simply remembered the image, rather than the actual historical facts. Charles Clarke noticed the error immediately, but Keats chose to leave it in, presumably because historical accuracy would have necessitated an unwanted extra syllable in the line.

Wikipedia