If you add the cpugraph-plugin to your panel and untick the option "Show current usage bars" the next time you log out an in to your session the plugin crashes. .xsession-errors shows the message: xfce4-panel-Message: Plugin cpugraph-10 has been automatically restarted after crash. The plugin then has to be removed and cannot be restarted. This applies to version 1.0.3
Seems that this can be reproduced on Debian (http://bugs.debian.org/679758)
I can reproduce it here, looking into it.
Fixed in http://git.xfce.org/panel-plugins/xfce4-cpugraph-plugin/commit/?id=25b2e1e8deec574201092559901d6154dc608ab7, will be in 1.0.5.
*** Bug 9083 has been marked as a duplicate of this bug. ***