Sorry if this has been dicsussed before, but I noticed that in Leopard, when you have 2 or more users logged in at the same time and try to shut down, you are asked for one of the user's username and password. Cool right? If you type it in correctly and hit enter, the computer shuts down fine. But if you try to actually click the "shut down" button, a window will come up saying that invalid username or password was entered. Try it out.