502 due to "too big response headers"

I'm seeing a 502 from Nginx when I request from `admin-ajax.php`, the error is:

`2013/08/23 13:58:52 [error] 18860#0: *1 upstream sent too big header while reading response header from upstream, client: 192.168.50.1, server: local.pencil.dev, request: "GET /wp-admin/admin-ajax.php HTTP/1.1", upstream: "fastcgi://unix:/var/run/php5-fpm.sock:"`

Is anyone else seeing this?

After some experimentation with [some suggestions found on Stack Overflow](http://stackoverflow.com/questions/11526674/nginx-big-header-response), the solution seems to be to to increase the `fastcgi_buffers` and `fastcgi_buffer_size` in Nginx.