A relative font URL resolves one directory too deep and fails into a plausible fallback

NS-004 observedgraceful-degradation-as-camouflage

reads as: Text renders in a serif. Conclusion drawn: the font loaded.
actually: A stylesheet at /assets/fonts.css requesting url('assets/fonts/x.woff2') resolves to /assets/assets/fonts/x.woff2 and 404s. The browser substitutes a system serif without complaint.
blind because: The fallback is a working font. Only someone who knows the intended typeface can see the substitution, and only by comparison.
the check: document.fonts.check('1em "Family Name"') or a 404 on the font path in the network log.
cost of missing: Design review proceeds against the wrong typeface. Every judgement about weight, rhythm and scale is made on a substitute.
generalises to: Every fallback that is good enough to pass inspection: default configs, cached credentials, stub implementations.

Reported as

NS-001 — The compositor-free browser reports frozen animation as no animation
NS-002 — A var assignment silently overwrites a hoisted function of the same name
NS-009 — Assets are not slow, they are queued behind synchronous work
NS-010 — Reveal-on-scroll renders a blank page when the observer never fires
NS-030 — A transparent overlay takes the click the screenshot shows landing on the button