Https causing redirect loop?

So, after hours of painful debugging we finally worked out the issue.

Nginx
Yep, the load balancer was doing it's own redirects.

In the nginx setup this line existed proxy_redirect http:// https://;

So, what was happening?

Well,My code was saying "this page should not be https, redirect"
Then nginx was saying "this request is not https, redirect"
Then my code was...
boom redirect loop.

So, by changing proxy_redirect http:// https://; to proxy_redirect Off; we fixed the problem :)

The function that you posted for disable_ssl() shows:

if($_SERVER['HTTPS'] == '0n') {

Is the 0 ('zero') a copy/paste error? I assume that you meant:

if($_SERVER['HTTPS'] == 'On') {

If I'm following your logic correctly, this function is invoked when you goto https://test.example.com, and so if this typo exists, it is probably not doing what you think it should.

We've addressed the accurate fix for Nginx (and other reverse proxies) here:

http://www.sonassi.com/knowledge-base/magento-kb/magento-https-redirect-loop/

This avoids the necessity to have that code in your index.php file. All you needed to do was add this to your Nginx config,

fastcgi_param  HTTPS on;