Following on from T240248: "PHP Notice: Undefined index: key" and similar in Cite.php and ReferenceStack.php

It'd be useful if errors ended up in some more (I know some exist already) specific categories. On enwiki, Category:Pages with reference errors has over 5,400 pages, nearly 24K on User pages, and over 9000 on Talk pages

[00:59:23] <James_F> I think the spammy line is particularly triggered by re-definition of a reference, specifically.