I have been seeing this since a while, but whenever I load a page on stackoverflow.com or meta.stackoverflow.com I see Unexpected response code: 502
in the console on Chrome.
It is innocuous for me, but I am guessing that these 502's are the result of extraneous calls which must add some server strain.
Update: seems that the WebSockets are to blame. From the HAR:
{ "name": "Host" , "value": "sockets-se.or.stackexchange.com" },
And when I visit sockets-se.or.stackexchange.com I get
502 Bad Gateway, The server returned an invalid or incomplete response.