When playing flash videos (youtube, vimeo, whatever) in full screen mode, using my keyboard shortcuts to adjust the volume always causes me to exit full screen mode. Unfortunately, they're my only way to adjust the volume, so it's really annoying.
SnapOverflow
This is a bug in the Flash player.
The volume keys on your keyboard are handled by gnome-settings-daemon, which uses a passive grab on the volume keys. The way that X works means that an application needs to have focus in order to receive keypress events, so, what happens when you press the volume keys is that the flash player window sees a FocusOut event as focus is moved temporarily to gnome-settings-daemon.
However, the FocusOut event has the mode set to NotifyGrab, and the flash player should be ignoring this event (it should only really respond when the mode is set to NotifyNormal, which would happen if you really did focus another window).
Hope that helps explain it a bit.
Chris's explanation was very good, and there is also a Launchpad bug with more information.