The simplest failure is the most common: the head never got its meta tags. No og:title, no og:image, no twitter:card. It happens because tags are invisible in the browser, so nothing in development or QA ever fails. The page looks perfect right up until someone shares it.
Each platform handles the silence differently, and that’s the trap. Facebook scrapes your title tag. iMessage might show a favicon. Slack shows the naked URL. The same link produces three different first impressions, none of them designed, and the fallbacks change whenever the platforms feel like it.
This isn’t a small-site problem. Hacker News, the internet’s watercooler, ships zero social tags on any page.