PHP Warnings end up with status code 500

[eluser]Unknown[/eluser]
Problem is the following: In CI environment, PHP warnings end up with status code 500 which means I can't see the error, just "Page doesn't exists" error by browser.

I've tried the following things:

- removed .htaccess file to rule out bad lines in there
- produced the same error in a file not connected to the CI (but in the same directory), error showed up fine, got logged by apache
- produced the error in a CI controller (only error producing, nothing else), ends up with 500, apache nor CI logs the PHP error
- deleted contents of error functions in Common.php under codeigniter, nothing changed (yes, also contents of set_status_header function, still nothing)
- deleted Exceptions.php file in Core, nothing changed
- Searched for "extends CI_Exceptions" from all the files in the project, found nothing
- php display_errors is set to 1 (checked, twice), php error_reporting is set to all (checked, twice)
- there are no custom hooks
- PHP syntax errors are showing up fine (don't end up 500 in CI environment)
- CI environment is defined to development
- CI install is not vanilla, CI not in the latest version

I'm using nginx + apache + CI setup, but if it would be apache + nginx setup problem, I'd also end up having 500 in files not connected to CI anyhow (other than the same directory)

ANY and I really mean ANY ideas what to look for or what to test would be welcome, I've ran out of ideas.

Here's the code I'm using to produce the PHP warning:
http://pastebin.com/8RR3qb5J

Help appreciated!

[eluser]Unknown[/eluser]
Found it!

Firephp.php in libs\ was catching errors and giving out header('HTTP/1.1 500 Internal Server Error'); for some reasons.

Got it fixed now