I must have written about this elsewhere, but here are my top three:
.DS_Store. Finder litters faster than one could clean up.
HFS+ NFD*.1 Heard of the cursed encoding
UTF8-MAC? Pure Evil. Culprit of tons of garbled text issues (especially cross platform ones) and probably most length miscalculation issues. Even Apple's Terminal.app can't do NFD right. I wonder how Korean users navigate their filesystems in terminal.
Plist XML. It's XML, but even worse.
NFD with an asterisk, i.e., not even NFD. According to Apple in an old Technical Q&A,
The terms used in this Q&A, precomposed and decomposed, roughly correspond to Unicode Normal Forms C and D, respectively. However, most volume formats do not follow the exact specification for these normal forms. For example, HFS Plus (Mac OS Extended) uses a variant of Normal Form D in which U+2000 through U+2FFF, U+F900 through U+FAFF, and U+2F800 through U+2FAFF are not decomposed (this avoids problems with round trip conversions from old Mac text encodings). It's likely that your volume format has similar oddities.
They are conscious enough to call these oddities.↩︎