Hello Ian !
Thanks for looking into the bug.
Regarding your question whether it happend once: yes and no, it happened several times last summer, but that was an older kernel version, debians linux-image-3.14-2-amd64 back then and self-built 3.15.7, both with xen 4.4.0
Will try to include more information regarding this bug later, from the "reportbug" application.
If there is any other application to be tested with, you can suggest it to me.
Kind regards,