Canonicals are how a site resolves its own duplicates: a print view, a filtered variant, the same product under two categories. Pointed correctly they consolidate signals onto one URL. Pointed wrongly they remove a page from search results, and nothing on the page looks broken.
The two failures worth watching for are a page canonicalised to something else by accident, and a page declaring one canonical in its HTML and a different one in its response headers, which leaves the choice to Google.
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