The planet Vulcan: a cautionary tale

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.

This all seems like science and scientists getting the job done, a tale of scientific discovery and triumph, not cautionary.

IIRC, there was the Planet X search ~100 years ago. Much later it was pointed out either there was a miscalculation of Neptune's mass or a slight issue with Newton's theory. Since then it we know that Planet X does not exist.

Now we have the search for Planet 9, details for both:

https://en.wikipedia.org/wiki/Planets_beyond_Neptune