- Start with NS_HELP_TALK not in $wgFlowOccupiedNamespaces
- Create Help talk:Foo (as a wikitext page)
- Add NS_HELP_TALK to $wgFlowOccupiedNamespaces
- Visit Help talk:Foo again
You'll see an error saying "Unknown topic. The requested topic doesn't exist".
This seems like it would be a problem for any full-namespace conversion, since we intend to skip conversion for archive pages (or more generally, subpages without corresponding subject namespace pages). This error would leave these archives (which could even have been created by Special:EnableFlow to begin with) inaccessible after conversion.