The subtlest failure is shipping some of the tags. It comes in flavours: Oreo sets og:title and og:description but no og:image, so its card collapses to a text row. Patreon ships the image but no og:title, so platforms improvise the headline from the tab title. Seventh Generation sets a title and nothing else. Chobani puts everything in twitter:* and nothing in og:*, so its links work on X and nowhere else.
All of these pass a glance test. The card renders something on the platform someone checked, and the missing piece only shows on the platforms nobody at the company uses. That’s why partial tags survive in production for years at brands with entire teams for this.
The rule that prevents the whole category: the tags are a matched set. Title, description, image, both namespaces, same values. Anything less means some platform somewhere is writing your copy for you.