Keats’ Ode on a Grecian Urn ends with these lines;
Beauty is truth, truth beauty,---that is all
Ye know on earth, and all ye need to know.
Eliot, in his essay on Dante, called the lines ‘a serious blemish on a beautiful poem’. Serious is the right word. A serious blemish is an earnest blemish, and an earnest blemish is almost an intentional one.
Earlier in the same poem, Keats wrote;
Bold Lover, never, never canst thou kiss,
Though winning near the goal yet, do not grieve;
She cannot fade, though thou hast not thy bliss,
For ever wilt thou love, and she be fair!
So completed poems are sweet, but those uncomplete are sweeter. By ending with the bitterness of false rhetoric, Keats demonstrates the truth of the poem at the cost of its beauty. While emphasising the falseness of the final equivocation, Keats boasts. He knew he had beauty to spare, so he chose to prove his theme with ugliness.
Edit: A Mouthfull of Air just released an episode on the poem, go listen to that instead.