The wrong ISO code that cost a market 40% of its clicks

The wrong ISO code that cost a market 40% of its clicks

Hypothesis: a single malformed region code was silently invalidating an entire market's annotations. Methodology: forensic case, not an experiment — we were brought in after a UK property lost ~40% of UK organic clicks over a quarter with no ranking-factor explanation. ~3,000 URLs.

Findings, traced step by step:
— The dev team had used en-uk instead of the correct en-gb. 'UK' is not a valid ISO 3166-1 region code; the correct code for the United Kingdom is GB.
— Google ignored the invalid annotation entirely, so the UK pages had effectively no hreflang. The US variant began outranking them for UK searchers on shared queries.
— We can date the decline to within a week of a deploy that introduced the bad code — strong circumstantial evidence, though not a controlled link.
— Fix was one find-replace across templates. UK correct-country serving recovered from ~58% to ~90% in 5 weeks; clicks recovered ~31% (not fully to baseline within the window).

Caveats: this is a single forensic case, and we can't fully exclude a concurrent factor. But the timing, the mechanism, and the recovery curve all point one direction.

Conclusion: en-uk is the single most common invalid code we see. It fails silently — no error, no warning, just slow market erosion.