I don't understand what makes this a cautionary tale. Observation didn't quite match theory. An entirely reasonable hypothesis was proposed: an undiscovered planet. Attempts were made to prove it by discovering said planet. They failed. Eventually, a genius came up with a new theory that explained the observation, and a whole lot of other stuff besides.<p>This all seems like science and scientists getting the job done, a tale of scientific discovery and triumph, not cautionary.
"They failed." is incorrect. For several decades the consensus was that they'd succeeded, with a few dissenting voices. It wasn't until Einstein came up with a theory that perfectly explained the precession of Mercury that it became obvious that the observations were bogus. That's the part which should make you cautious, both when believing other people's observations and when making observations yourself.