Fascinating stuff. (And by fascinating, I mean terrifying.)

A good example is Progesterone.

Progesterone is a naturally occurring steroid hormone and is used in hormone therapy and birth control pills, among other applications. There are two known forms of naturally-occurring progesterone (or nat‐progesterone), and other synthetic polymorphs of the hormone have also been created and studied.

Early scientists reported being able to crystallize both forms of nat‐progesterone, and they could convert form 2 into form 1 (which is more thermodynamically stable and melts at a different temperature). When later scientists tried to crystallize form 2 from pure materials, they could not. Attempts to replicate older instructions (and variations on those instructions) for crystallization of form 2 invariably produced form 1 instead, sometimes even leading to crystals of exceptional purity but still of form 1. Researchers have tentatively suggested that form 2 became gradually harder to produce around 1975, based on a review of production difficulties documented or alluded to in existing literature.

wiki link: https://en.wikipedia.org/wiki/Disappearing_polymorph