I worked for a company where a proxy server was used for all internet access. Every now and then a pages would not load. Error logs pointed me to the usual culprit - dns. When looking at dns traffic in tcpdump everything looked normal, except some dns replies came from rfc1918 addresses instead from the dns servers public IP address. When I talked to the ISP, they blamed me (the proxy) for reusing UDP sockets, and it was by design that their load balancer would only support one DNS request at a time per UDP socket. If there were two or more in flight, only the first response would be NATed properly. Luckily, I knew that the ISP used the same proxy product internally, so when I asked how they configured their proxy to avoid this issue, they fixed the load balancer within the hour