[1] [CITE@en-US[Error 520 – CloudFlare Support]]
( ([TIME[2014-05-14 07:33:42 +09:00]] 版))
<https://support.cloudflare.com/hc/en-us/articles/200171936-Error-520>

[2] [CITE@en[520 (Origin Error) in AJAX calls · Issue #57 · johnbillion/query-monitor]]
( ([TIME[2015-01-14 11:13:39 +09:00]] 版))
<https://github.com/johnbillion/query-monitor/issues/57>

[3] [CITE@en[Getting a lot of HTTP 520 errors. What to do with them > – Customer Feedback for Microsoft Azure]]
( ([TIME[2015-01-14 11:13:44 +09:00]] 版))
<http://feedback.azure.com/forums/223579-azure-preview-portal/suggestions/5965429-getting-a-lot-of-http-520-errors-what-to-do-with>

[4] [CITE@en-US[Understanding HTTP 520 Error – Proxy Publisher Failure | Code Web]]
( ([TIME[2015-01-14 11:13:48 +09:00]] 版))
<http://codeweb.in/understanding-http-520-error-proxy-publisher-failure/>