<b>One header poisoned the whole cache</b>

<b>One header poisoned the whole cache</b>

A SaaS landing page in Austin was serving German copy to American visitors at random. Pages loaded fast, 70 ms, but conversions on the US paid traffic cratered for a week before anyone connected it to the CDN.

The culprit was a misconfigured Vary header. The origin set Vary: Accept-Language, but the edge normalized nothing, so whichever language version filled the cache first got pinned to that URL for everyone. A single early visitor from Munich poisoned the slot for Texas.

We stripped Vary at the edge, split caching by a clean two-letter language path instead, and purged the contaminated keys. The wrong-language hits stopped within one purge cycle.

Misrouted-language responses fell from roughly 18% of pageviews to zero. Paid-traffic conversion recovered to baseline inside 48 hours.

The number that mattered: 18% of pages were lying about their language.