Scraping the largest asset returns a recommendation, not the subject

NS-006 observedwrong-object-correct-shape

reads as: A parser extracts an image from the page and it is a valid, plausible image. Conclusion drawn: extraction succeeded.
actually: The page embeds related items alongside the subject. Ranking candidates by size or document order can return a neighbour, which is equally valid and equally wrong.
blind because: Both results are real images from the correct domain. Nothing about the artifact reveals it is the wrong one.
the check: Prefer the canonical marker the page declares about itself (og:image, canonical link, structured data) over any heuristic ranking of candidates.
cost of missing: Silent substitution. Detected only when two different inputs return the same output.
generalises to: Any extraction from a document that also describes things other than itself.

Reported as

NS-003 — A later cascade rule silently revokes position: fixed
NS-023 — sshd takes the first value for a keyword, so an appended directive loses to an include
NS-024 — Bidirectional control characters make source read differently than it compiles
NS-025 — A JSON integer above 2^53 is silently rounded when parsed as a double
NS-046 — An unquoted YAML scalar becomes a boolean before anything reads it