Comment 88 for bug 439448

Exactly the point many bug reporters have been making. Although this doesn't cause a serious crash, it can effectively lock an inexperienced user in to their system with no way of exiting. Sadly, this issue highlights a major flaw in the Ubuntu bug reporting process, in that the importance of an issue is viewed differently by different people depending on their technical background......

As a work around for the issue you had, and this if for new users stuck with this issue until a proper fix is released:

1) Right click on the panel (the top bar) and choose Add to panel
2) Scroll down the list of options and choose Shut Down
3) This will add a nice red Shut Down button to your panel, giving you the option to shutdown or restart.
4) For extra safety, right click on the newly added red Shut Down button and select move.
5) Drag the button to the right hand side of your panel, this way it is least likely to be affected by these graphical issues.

This is the only reliable GUI way I have found to shut down the system (Yes, I know I could simply type "sudo shutdown -hP now" in the terminal, but these days Ubuntu is also aimed at the average non-techie user coming from Windows, hell, it is even sold on Dells now, so we can't expect all Ubuntu users to know about this....)