Sorry, Mutter closed unexpectedly on Ubuntu 10.10

I am just exploring and understanding various features on Ubuntu 10.10 netbook. I install it on a usb stick and running from it. I am not sure what cause this problem. But I think after closing a window suddenly a message appear and says

Sorry, Mutter closed unexpectedly

This message box also include a request. It say

If you were not doing anything confidential (entering passwords or other private information, you can help to improve the application by reporting the problem.

Mutter is a window manager for ubuntu netbook edition. I am not sure why it happened. Is it a bug?