When the Oneboxer is blocked by CloudFlare, it would be a much better user experience if instead of:
Sorry, we were unable to generate a preview for this web page, because the following oEmbed / OpenGraph tags could not be found: image, title
Discourse revealed the actual problem to users:
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Concept screenshots:
(before)
(after)
This could be detected by the presence of a cf-mitigated: challenge header in the 403 response as per Detect a Challenge Page response · Cloudflare challenges docs