XMLHttpRequest cannot load http://m.agar.io/?_=1430388662333. No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://agar.io' is therefore not allowed access. The response had HTTP status code 520.
So if for instance your page makes a CORS request and the MySql server is away. Your app framework catches the error and renders the 500 message, but it doesn't automatically set the CORS.
Now your page receives a response that just gets flat out rejected, you can't even inspect it in the network tab.
The easiest solution is probably to set the CORS headers in NGINX.