A canonical tag tells Google "this is the real version of this page." When several unrelated pages all point that tag at the same other URL, it's rarely intentional — and it can quietly remove those pages from search results.
The canonical tag (<link rel="canonical" href="...">) exists to handle genuine duplicate or near-duplicate content — telling search engines "index this URL, not that other near-identical one." Used correctly, each page's canonical tag points to itself, unless it's a deliberate duplicate of another specific page.
When a crawl finds several completely different pages — different topics, different content — all canonicalizing to the same single other URL, that's not normal duplicate-content handling. It's almost always a leftover staging configuration or a CMS/plugin default that got applied sitewide without anyone noticing, often pointing every page back at the homepage or a template default.
The practical effect: Google is being told, sitewide, "none of these pages are the real version — index that other one instead." Pages can quietly drop out of search results even though nothing about their actual content changed.
Canonical misconfigurations across multiple pages are one of the checks we run automatically.
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