Find a non-existent Flow board e.g. a user talk page on ee-flow. Click the pencil to edit the header, then click [Save header]
You get a pink "An error occurred while contacting the server." errorbox. On ee-flow, the apache error.log contains:
[Sat Jan 04 04:48:56 2014] [error] [client 216.38.130.164] PHP Fatal error: Call to a member function isAllowed() on a non-object in /srv/mediawiki/extensions/Flow/includes/Block/Header.php on line 83, referer: http://ee-flow.wmflabs.org/w/index.php?title=User_talk:4pq1injbak&action=edit
Just before this, I think upon visiting the page, there's a warning in apache error.log:
[Sat Jan 04 04:58:07 2014] [error] [client 216.38.130.164] PHP Warning: header() expects parameter 3 to be long, string given in /srv/mediawiki/includes/WebResponse.php on line 38, referer: http://ee-flow.wmflabs.org/w/index.php?title=User_talk:4pq1injbak&action=edit
Note there's a separate bug 58018 about the different error you get trying to preview the header change on a non-existent page.
Version: master
Severity: normal