Bug 60834 complains that ?action=info doesn't work on a Flow page. Perhaps action=info should display useful information, see discussion at that bug.
The issue at hand is whether Flow should ignore unrecognized/unsupported actions, as it does now -- you just see the Flow board. That's misleading if someone follows a URL that used to do something useful before Flow was enabled on the page.
On a regular wiki page ?action=BAD results in
No such action
The action specified by the URL is invalid. You might have mistyped the URL, or followed an incorrect link. This might also indicate a bug in the software used by {{SITENAME}}.(enwiki customizes the second, <nosuchactiontext>, message).
Maybe Flow should respond similarly, with a <flow-nosuchactiontext> changed to "... is invalid for a Flow board. ..."
Version: master
Severity: normal
URL: https://www.mediawiki.org/wiki/Talk:Sandbox?action=BAD
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=62173