I have some strange behaviour with my xfce-4.10 installed with Xubuntu 12.10 (I've also noticed the same behaviour with an Arch/xfce installation).
I have two panels, one across the top of the screen, the other down the left hand side. I've set both to 'autohide' and have the action buttons applet at the right hand end of my top panel, set up to display the session menu, rather than the action buttons.
If I click on the session menu and then cancel it without selecting any of the options, then the top panel remains visible when I move the mouse off it, but any windows behave as if the panel is hidden. Most of the time, if I switch to action button display, then the autohide starts working again and remains working if I switch back to session menu, but sometimes this doesn't work.
Has anyone else noticed this behaviour? I've had a search of the forum and bugzilla, but didn't see anything.