<b>The site caching desktop and mobile as the same broken page</b>

<b>The site caching desktop and mobile as the same broken page</b>

A classifieds site used edge-side device detection to serve different markup to mobile and desktop — but forgot to add the device class to the cache key. Whichever version was fetched first got cached and served to everyone. Half of London's mobile users were getting the desktop layout, and half of desktop users got the cramped mobile one, depending on a coin-flip of who hit the cold edge first.

The fix was a one-line cache-key change: include the normalized device type (<code>mobile</code> / <code>desktop</code>) so the edge stores exactly two variants per URL — not one shared mess, and not a fragmented copy per device model.

Layout-mismatch support tickets dropped to zero. Cache-hit ratio held at 95% because only two clean variants existed. Mobile bounce rate in London fell 22% once users stopped landing on a desktop page.

The number that mattered: 2 correct cached variants instead of 1 wrong one.