In OS X, as a limited user I used Terminal to
su - [admin's name] and tried to move some files that were in the administrator's public drop box directory into /usr/bin. Couldn't do it. Permission denied. The Terminal prompt was clearly in the admin's login shell/environment.
Then, I discovered that, from the limited user's login, I could use the menu system to navigate into /usr/bin. I could then move the same files (copies in my directory, not the admin's copies) into /usr/bin as long as I authenticated as admin.
Seems backward. How is it that a limited user could get into the /usr/bin directory with administrative authentication, but the admin--at least a user in the admin's shell--could not?