collapse caused by screenshot

[马梓扬 <yagood ma gmail com>]

Today when I was visiting a website and wanted to take a screenshot, I
pressed Shift+Print Screen. Then I changed my mind and right-clicked
the mouse to cancel the screenshot. After I did this, however, the
gnome-shell collapsed and there came out a notification asking me to
send messages to ubuntu. A moment later the gnome-shell restarted
again. So I tried again the operations described above and the same
thing happened, except that gnome-shell didn't restart at this time.
What was worse, I couldn't input any more! That means I couldn't
restart my computer in the terminal. So I used tty2 to restart and
reported the problem here, wondering if this is a bug? Or the conflict
with other programs? The distribution I use is Ubuntu 12.04.1, and the
gnome-shell's version is 3.4.1.

Hope my messages do some help. Thank you for reading!