Interestingly, the original Macintosh File System (MFS, the predecessor to HFS which was the predecessor to the current HFS+) was a non-hierarchical filesystem

Interestingly, the original Macintosh File System (MFS, the predecessor to HFS which was the predecessor to the current HFS+) was a non-hierarchical filesystem (that's why HFS has the H). The original Macintosh system software supported hierarchical directories, of course - by maintaining a database mapping file-system nodes to paths. When the user opened a particular folder, it queried the database for nodes whose path matched. Of course, no database is perfect, and sometimes things got messed up. There was a special key chord the user could hold down during startup that would cause the Finder to rebuild all its databases. When done on an MFS volume, all the hierarchy painstakingly created by the user would be erased, and all the files on a volume would wind up in the root directory.