A while ago I came back to my home computer to find I had not logged out and things not as I left them. There were files on the Desktop with names like 'sdfgasfgafasz' and things moved or disappeared. It took this happening a couple of times before I connected it to my my three year old daughter and resolved to do something about it.
In OSX there are many ways ways to protect your login when you leave the computer, logging out, using a screen saver with a password etc. These all tend to have a shortcut (like ⌘⇧Q to log out of the current user or hot corners) but none of these quite fitted my usage.