502 Bad Gateway error

/var/log/nginx/error.log

06:50:11 [error] 861476#861476: *1 upstream sent too big header while reading response header from upstream,

Fix when Nginx is running in a proxy / reverse proxy mode

...
server {
...
 proxy_busy_buffers_size   512k;
 proxy_buffers   4 512k;
 proxy_buffer_size   256k;
...
}
...

Dealing with proxy_pass and fastcgi buffers

The error states the issue. fastcgi_busy_buffers_size is larger than fastcgi_buffers.

Append the following directives after fastcgi_pass:

...
location ~ [^/]\.php(/|$) {
...
    ## TUNE buffers to avoid error ##  
    fastcgi_buffers 16 32k;
    fastcgi_buffer_size 64k;
    fastcgi_busy_buffers_size 64k;
...