On the beta cluster, the page "User:Jackmcbarn/Page" contains "{{A nonexistent template}}" as the content. If you go to http://en.wikipedia.beta.wmflabs.org/w/index.php?title=Template:A_nonexistent_template&action=edit and enter "User:Jackmcbarn/Page" in the page title box, a fatal error will be generated when previewing: PHP fatal error in /srv/common-local/php-master/extensions/TemplateSandbox/TemplateSandbox.hooks.php line 205:
Call to a member function isRedirect() on a non-object
Version: unspecified
Severity: major