The application is Node.js based web crawler that does requests to remote servers via `request` library.

Requests are made through authenticated HTTP proxy servers with 99% and higher uptime, provided by third party vendor.

I'm trying to process HTTP codes for request errors response that can be likely caused by malfunctioning proxy. It's expected that after reaching a threshold of these HTTP codes, a flag that a proxy is unreliable will be triggered, so a proxy will be queued for diagnostics. And I expect some codes to have more weight.

Currently I'm processing 407 (proxy authentication) and 408 (request timeout) HTTP codes for proxies.

Are there other HTTP codes that are commonly caused by malfunctioning HTTP proxies?

I hope the question is specific enough. I assume that possible malfunction (and a set of possible HTTP error status codes) depends on actual proxy and its environment. We should consider these proxies black boxes because they are provided by a third party.

**The question isn't about HTTP status codes in general, only error codes can be caused by malfunctioning proxies (forward proxies).**