Comment 1 for bug 844809

Reproduced this behavior on Precise in Unity as well.

I put nautilus on one display, and gedit on the other, both maximized. On the display with nautilus (the external monitor), using the mouse to navigate the various indicator menus works. Using the arrow keys to do the same likewise works. On the other display (the laptop lvds), mousing through the menus works, but when I use the left/right arrow keys I see an empty menu box pop up, which has arrows at the top and bottom and appears to be sized for content but no content is visible. Also it is positioned at the far top right corner of the screen rather than beneath its indicator.

Further experimentation shows it behaves like this even if all applications are shut down.