Thank you for raising the question.
In some field guides you will see a species called Squamanita umbonata (Sumst.) Bas. At one point in the history of this strange fungus, it was assigned to Amanita [as A. umbonata (Sumst.) Sartory & L. Maire]. This was before Pomerleau thought of naming A. umbonata Pomerleau.
Hence, Pomerleau had unknowlingly created a posterior homonym. Such a name cannot be accepted as correct. Hence, when Pomerleau learned that there had been a previous use of the combination “Amanita umbonata,” he had to change his name to a new one and picked the epithet “jacksonii.”
This story falls into the (large) “stuff happens” category of mycological nomenclature.
Very best,
Rod
