Disappearing polymorphs, also known as perverse polymorphism, refer to the phenomenon in materials science where a stable crystal structure suddenly transforms into a polymorph, or a different crystal structure with the same chemical composition, during nucleation. This transformation is often irreversible and may be caused by contamination with microscopic seed crystals, making it difficult to recreate the original polymorph. Disappearing polymorphs can have significant implications for industries like pharmaceuticals and computer hardware, where product effectiveness and manufacturing processes can be compromised. A few notable examples include the drugs paroxetine and ritonavir, which have faced legal and manufacturing challenges due to unexpected polymorphism. Despite their disappearance, it is believed that the original polymorphs can still be reconstructed, albeit with difficulty.
https://en.wikipedia.org/wiki/Disappearing_polymorphs