Sometimes my Windows 7 login screen doesn't show up when I wake up the computer after the screen has shut off. Note that I mean the screen turning off from X minutes of inactivity, not the computer entering sleep mode. When I move the mouse to turn the screen back on, it shows up right at my desktop. This seems to occur in "waves", so it will work fine for a month or two, then suddenly it will stop displaying the login screen for several weeks or so, then it will go back to showing it. I have no idea what is causing this change.
The only thing that I suspect could possibly be the culprit of this is TeamViewer, since it sometimes (but not others?) locks the computer after leaving a session.