A server (not necessarily a Web server) is acting as a gateway or proxy to fulfil the request by the client (e.g. your Web browser or our CheckUpDown robot) to access the requested URL. This server received an invalid response from an upstream server it accessed to fulfil the request.
This usually does not mean that the upstream server is down (no response to the gateway/proxy), but rather that the upstream server and the gateway/proxy do not agree on the protocol for exchanging data. Given that Internet protocols are quite clear, it often means that one or both machines have been incorrectly or incompletely programmed.
This problem is due to poor IP communication between back-end computers, possibly including the Web server at the site you are trying to visit. Before analysing this problem, you should clear your browser cache completely.
If you are surfing the Web and see this problem for all Web sites you try to visit, then either 1) your ISP has a major equipment failure/overload or 2) there is something wrong with your internal Internet connection e.g. your firewall is not functioning correctly. In the first case, only your ISP can help you. In the second case, you need to fix whatever it is that is preventing you reaching the Internet.
If you get this problem for only some of the Web sites you try to visit then it is likely to be a problem at those sites i.e. one of their pieces of equipment is failing/overloaded. Contact the people at those sites.
Our service monitors your site for HTTP errors like 502. Use of proxies and caching is increasing on the Web. Our CheckUpDown robot will always try to "drill through" to the real computer that actually hosts the Web site, but we do not have complete control over where our HTTP request actually ends up. If only one link in the chain of computers dealing with our HTTP request is broken, then an error such as 502 can easily occur.
Please contact us (email preferred) whenever you encounter 502 errors - there is nothing you can do to sort them out. We then have to liaise with your ISP and the vendor of the Web server software so that they can review the flow of IP data traffic between various computers under their control.
Any client (e.g. your Web browser or our CheckUpDown robot) goes through the following cycle when it communicates with the Web server:
This error occurs in the final step above when the client receives an HTTP status code that it recognises as '502'.