PlanNine has the concept called "Name Spaces" where children inherit the filesystem from their parents, but are allowed to change it for them and all of their children. Thus, if you "mount" a file system in one session, then it may not be available in another session.

This was quite cool, since when you logged in, it mounted your home directory over /, thus giving you your own "filesystem" to play with, but any changes you made actually went into your home directory. The window manager 8(1/2) munged the filesystem in each "window" to have a "/dev/fb" which pointed to the framebuffer of that window.