About 0.5% requests are returning 499 with the attached Instant Log (see image). Other requests are 200 OK. What can cause this kind of client failure?
What feature, service or problem is this related to?
This is nginx specific response code to indicate when the connection has been closed by the client while the server is still processing its request, making the server unable to send a status code back.
Since Cloudflare was built on nginx, we also have a 499 HTTP code in Cloudflare Logs and analytics for connections which are terminated before we have finished processing the request. It is expected behavior to see these in your logs intermittently as clients close connections.