Part 1
Visit https://www.mediawiki.org/wiki/Talk:Sandbox?workflow=abcd , the h1 and page title are
Internal error - {{SITENAME}}
Appending &uselang=qqx suggests the message is
(pagetitle: (internalerror)) (flow-error-invalid-action)
So the pagetitle message is looked up, but isn't getting parsed.
Part 2
Visit https://www.mediawiki.org/wiki/Talk:Sandbox?action=FOO , the h1 and page title are the same Internal error - {{SITENAME}}
Flow's InvalidActionException->getErrorPageTitle() is returning the better title 'nosuchaction' for this error, but it isn't appearing.
Version: master
Severity: normal
URL: https://www.mediawiki.org/wiki/Talk:Sandbox?action=FOO