Comment 3 for bug 279775

Furthermore, if the Virtualbox window was active when the screensaver activates, you need to press the host key (right ctrl by default) in order to bring up the screenblanker's password dialog. This is highly startling because until you realize what's going on and how to work around it, it looks like there is simply no way to disable the screensaver and log back in (potential data loss if the only workaround you come up with is a panic reboot).