See other failures under 000.html.
Broken derailleur hangar. Cause of failure unknown. From http://gallery.zed1.com/broken-bike as of 2008/07.
Note that non-removable derailleur hangars are typically more durable than removable hangars; but on failure the removable one can be replaced more easily. Damage to non-removable hangars is sufficiently common that the more fragle removable hangar may be a good choice for many.
See also FAIL-088.html.