The "HTTP 502 Bad Gateway" error occurs when a server acting as a gateway or proxy receives an invalid response from an upstream server. This means that the server you are trying to access is not able to communicate properly with another server that it relies on to fulfill your request. Common causes of a 502 error include server overload, network issues, or misconfigurations in the server settings. It can happen with various web services and applications, affecting users' ability to access websites or online resources, such as those hosted on Apache or Nginx servers.