An issue occurs if some page is regularly published and thus indexed in Solr. If later on editors decide such page should not show in search results anymore, they have a checkbox for this. When the page is rendered again, it gets robots:noindex attribute and also is no longer rendered in sitemap (which is used as entry point for Solr crawler). However, that means page stays indexed in Solr and will never be indexed again to update some data or even deleted.