When turning on output buffering and setting a call back function like:
	ob_start('callback_function');
The function is called right away. Not just in the end of the script.

Try this sample:
	ob_start('ob_gzhandler');
	ob_end_clean();

	echo "Step 1<br>\n";
	flush();
	sleep(1);

	echo "Step 2<br>\n";
	flush();

Function ob_end_clean should cancel the buffering and it dose, but ob_gzhandler is already called. It is called when ob_start is turned one. Now the script fails because the function ob_gzhandler has already set Content-Encoding header (meaning the browser expects compressed content).

A simple one:
	function callback_function($content) {
		die("Stop!");
		return $content;
	}
	ob_start('callback_function');
	ob_end_clean();

	echo "Step 1<br>\n";
	flush();
	sleep(1);

	echo "Step 2<br>\n";
	flush();

The script is stops before ob_end_clean is called.

I think that the problem is general. I have tested the problem on more than one computer.
The newest installation was a preinstalled Red Hat with a PHP version 4.3.1