<b>The most common hreflang error is an invalid code — and it's not what you'd guess</b>

<b>The most common hreflang error is an invalid code — and it's not what you'd guess</b>

We tabulated annotation errors across roughly 9,000 hreflang entries from real estates. The single largest error class wasn't reciprocity. It was malformed language/region codes.

Methodology: parsed every <code>hreflang</code> value against ISO 639-1 (language) and ISO 3166-1 Alpha-2 (region), flagging non-conformances.

Findings, by frequency:

— Region codes used where a language was meant. <code>en-UK</code> is the classic: the country code for the United Kingdom is GB, not UK. <code>en-UK</code> is silently invalid and ignored.

— Underscores instead of hyphens (<code>en_US</code>). The spec requires a hyphen; the underscore form is a locale convention copied from developer code.

— Inventing region codes that aren't ISO — <code>en-EU</code>, <code>es-LATAM</code>. There is no ISO country code for the EU or Latin America. These are dropped entirely.

— Capitalization is forgiving (Google normalizes), so that one rarely actually breaks.

Nuance: an invalid code doesn't error loudly — the entry is simply discarded, so the cluster silently loses a member and reciprocity breaks downstream.

Limitation: our sample skews toward estates that hired auditors, so error rates here likely understate the wild population.

Conclusion: validate codes mechanically before deploy. <code>en-GB</code> not <code>en-UK</code>; hyphens not underscores; real ISO regions only. No 'EU', no 'LATAM', no 'UK'.