@tsaukpaetra that reminds me, someone around here said that macOS stored in UTF-8 but it used the denormalised form rather than the normalised form.
This would certainly qualify too.
Why the hell would it matter?
Filenames are references and may be used across system boundaries. There are two normalized forms and if your filesystem forces one of them there will be when you interface with systems that don't.
Shirley every filesystem API therefore decomposes it's inputs.
They don't all. It's an Apple speciality.