I run VirtualBox in many environments and in Ubuntu this is the only place where I have come across this bug
I've installed the latest VirtualBox and this problem still persists, it's odd
Basically it's when you have a VM open with the display as full screen when you click on the minimise button on the options menu, the place where you can select to install guest additions from you see the VM minimise animation go off to the launcher but then when you move the mouse focus goes back to the minimised VM once you click on it again focus goes back to your desktop
I'm currently running Ubuntu 13.04 but I happens with 12.10 as well
If you need further detail maybe I'll be able to post a video of it, I think what I'd like to know is anyone else having this issue?