"Sorry Ubuntu * has experienced an internal error" /usr/bin/compiz
Solution 1:
It just means there's been a problem with compiz, which is the window manager on Ubuntu. If you don't experience any problems, then it has corrected itself. However, you can file bugs if you want to tell developers what happened so that they can fix it.
If your desktop is working properly, you don't have to worry about it. But you'll probably want to install Virtual Box Guest Additions, which will provide drivers for your graphics and other things.