Since March 14th Codex is sometimes returning a 500 error. It seems like this only happens if the wporg_logged_in cookie is set.
➜ ~ curl -v --cookie "wporg_logged_in=fooo%7C123456789%7C25ce9fd05b1aewf00509d6g5a602ae23" "https://codex.wordpress.org/index.php?title=Special:UserLogin&returnto=Help%3AContents" * Trying 66.155.40.241... * TCP_NODELAY set * Connected to codex.wordpress.org (66.155.40.241) port 443 (#0) * TLS 1.2 connection using TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 * Server certificate: *.wordpress.org * Server certificate: Go Daddy Secure Certificate Authority - G2 * Server certificate: Go Daddy Root Certificate Authority - G2 > GET /index.php?title=Special:UserLogin&returnto=Help%3AContents HTTP/1.1 > Host: codex.wordpress.org > User-Agent: curl/7.51.0 > Accept: */* > Cookie: wporg_logged_in=fooo%7C123456789%7C25ce9fd05b1aewf00509d6g5a602ae23 > < HTTP/1.1 500 MediaWiki exception < Server: nginx < Date: Fri, 17 Mar 2017 10:40:10 GMT < Content-Type: text/html; charset=utf-8 < Transfer-Encoding: chunked < Connection: keep-alive < X-Content-Type-Options: nosniff < Strict-Transport-Security: max-age=31536000 < <div style="border:1px solid #ffd0d0;padding:1em;">This is a cached copy of the requested page, and may not be up to date.</div><hr /><h1>Sorry! This site is experiencing technical difficulties.</h1><p>Try waiting a few minutes and reloading.</p><p><small>(Cannot contact the database server)</small></p><hr /><div style="margin: 1.5em">You can try searching via Google in the meantime.<br /> <small>Note that their indexes of our content may be out of date.</small> </div> <form method="get" action="//www.google.com/search" id="googlesearch"> <input type="hidden" name="domains" value="http://codex.wordpress.org" /> <input type="hidden" name="num" value="50" /> <input type="hidden" name="ie" value="UTF-8" /> <input type="hidden" name="oe" value="UTF-8" /> <input type="text" name="q" size="31" maxlength="255" value="" /> <input type="submit" name="btnG" value="Search" /> <p> <label><input type="radio" name="sitesearch" value="http://codex.wordpress.org" checked="checked" />Codex</label> <label><input type="radio" name="sitesearch" value="" />WWW</label> </p> * Curl_http_done: called premature == 0 * Connection #0 to host codex.wordpress.org left intact </form></body></html>