Well, I don't know if I should call it Bad Astrology (which is kind of tautologic), Bad Astronomy, or Bad Chronology, but it is an astronomical anachronism used in an astrological conversation in an otherwise quite good and accurate book.
"The Archer's Tale" (aka "Harlequin"), page 375:
This quote comes from an astrologer at the eve of the battle of Crécy, in the middle of the 14th century...

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Oops!

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He must have darned good eyesight.

Or else he drew up a horoscope so accurate and detailed it even predicted they discovery and subsequent naming of the next two planets!

Well, if astrology really worked, you'd think they could predict planets before the astronomers do.

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Of course, but to get even the name right is really a marvelous feat

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But in the 14th century, the astrologer was probably speaking French. He obviously said 'Poseidon' but it was changed in translation.

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I know you're joking, but I like irrelevant nitpicking, so I don't see why the French would use Poséidon (Greek) instead of Neptune (Roman, originally Etruscan).

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