Category:Pages with reference errors

When MediaWiki detects an error with the use of the <ref> tag, it assigns the page to this automatically created category. Some reference errors will show up in red text on the page with the problem, but there are also invisible errors that can get a page added here. To diagnose the issue, see Wikipedia – Cite errors for a list of possible causes. But also see this one weird fix which I had to make to remove the page from this category (this seems to be an undocumented bug as of MediaWiki 1.39, but I couldn't reproduce it on Wikipedia under MediaWiki 1.44, so it might have been fixed).