Another very interesting thing that you can do with filesystems is completely abuse the assumptions about what a filesystem can do, and thus come up with strange and unusual filesystems. (Cue Twilight Zone theme music...)
The easiest entity to abuse is the symbolic link. Effectively, it's a back door into the filesystem. Since you control what happens in the c_link() entry point when the symlink is created, you control the interpretation of the symlink. This gives tremendous potential for abuse—as I like to quote Dr Seuss:
Then he got an idea!An awful idea!
The Grinch got a wonderful, awful idea!