When a page with a <pages /> tag refers to a nonexistent Index: page, it spits out a visible big red error message: "Error: No such index". I just found a page with that error that had been sitting there since 2014 when the Index: was deleted!
It'd be nice if ProofreadPage also categorized such pages into a maintenance category so they'd be easier to discover. Something like "Category:Page transclusions that refer to missing Index files" or whatever.
Change 746839 had a related patch set uploaded (by Inductiveload; author: Inductiveload):
[mediawiki/extensions/ProofreadPage@master] Add a category for <pages/> referring to a missing index
Change 746839 merged by jenkins-bot:
[mediawiki/extensions/ProofreadPage@master] Add a category for <pages/> referring to a missing index
Change 891583 had a related patch set uploaded (by Tacsipacsi; author: Tacsipacsi):
[mediawiki/extensions/ProofreadPage@master] Make proofreadpage_nosuch_index_category a real tracking category
Change 891583 merged by jenkins-bot:
[mediawiki/extensions/ProofreadPage@master] Make proofreadpage_nosuch_index_category a real tracking category