A bad topic URL like http://ee-flow-extra.wmflabs.org/wiki/Topic:rxe303j17zih40ul causes an exception and resturns status 400 Bad request.
Since these are now the external URLs of permalinks, a bad topic should have friendlier failure message (c.f. https://www.mediawiki.org/wiki/Thread:Extension_talk:Flow/Badthread?uselang=qqx ) and return 404. It would be useful to track the referrer of broken links, I dunno if WMF does this in general.
Version: master
Severity: normal