We have one workstation that seems to refuse to let people log-off. You can click on "Log off" and the taskbar and desktop icons will disappear, but it will just stop there.
It doesn't freeze though. Pressing ctrl+alt+del and then 'cancel' will then bring back all the icons and the taskbar. So far, the only way I have been able to log the computer off is to do it remotely or by forcing a shutdown (not something I like to do very often).