The Wisdom Of The Immortals

Poul Anderson's mutant immortal, Hanno, prefers not to relate that he was at the court of a post-Roman British warlord who resisted English invaders and was called Artorius because "'...in this case the truth looks too much like invention...'" (The Boat Of A Million Years, p. 229)

Neil Gaiman's immortal Englishman, Hob Gadling, is:

"Old enough to have learned to keep my mouth shut about seeing a bloody great snake in the middle of the ocean."
-Neil Gaiman, The Sandman: Worlds' End (New York, 1994), p. 89.

So the people who know the truth about King Arthur and the Sea Serpent do not tell anyone!

Hob tells the cabin boy, Jim, who he can see is a girl:

"Given time, you'll spin a yarn of what we saw in the ocean. Given time I'll tell the tale of the handsome cabin boy. But given enough time and the right audience, the darkest of secrets scum over into mere curiosities." (p. 90)

And that is the advice that Manse Everard gives to younger Time Patrol members when they have openly intervened in the ancient world.