Update:
ob_flush() doesn't change anything ...
I accidentally tested it with display_errors=ON

You have a parse error there:

PHP Parse error:  syntax error, unexpected T_STRING, expecting T_VARIABLE in /home/jani/t.php on line 7

And I don't get any E_STRICT notices with fixed version of this code either..

Sorry .. 
the code example goes in the wrong direction 
I've narrowed the actual issue. It has to do with evaled code.
The @eval-Function is not catched properly.

<?php
error_reporting(E_ALL & ~E_NOTICE);

class Test {
    function func1() { // HTTP 500                                                                                                                                   
        ini_set('display_errors', 'off');
        @eval('completely wrong');
        echo 'boo'; exit();
    }
    function func2() { //HTTP 200                                                                                                                                    
        ini_set('display_errors', 'on');
        @eval('completely wrong');
        echo 'boo'; exit();
    }
    function func3() { // HTTP 200  and HTTP 500 with output buffering                                                                                               
        ini_set('display_errors', 'off');
        echo 'noerror';
        @eval('completely wrong');
        echo 'boo'; exit();
    }
}

Test::func1();
//Test::func2();                                                                                                                                                     
//Test::func3();                                                                                                                                                     
?>

That's expected behavior. 

In PHP 5.2.4 NEWS:

"Changed error handler to send HTTP 500 instead of blank page on PHP errors. "

And in the manual page for eval(): http://docs.php.net/eval

"If there is a parse error in the evaluated code, eval() returns FALSE and execution of the following code continues normally. It is not possible to catch a parse error in eval()  using set_error_handler()."

"Changed error handler to send HTTP 500 instead of blank page on PHP
errors. "

1. I've never seen a blank page in PHP4, when i'm doing @eval !
2. Why does it work with display_errors on?
3. Why does it work, when i'm doing an "echo" BEFORE the @eval?

Why bogus?

This is by no means a consequent error-reporting.

I've ran into the same problem on 5.2.9. Because of an error in the eval'd code(and the subsequent 500-Internal error generated), the headers i was sending from my app would get misinterpreted by the browser and would display a "File Not Found"...
I tracked this to a combination of "display_errors off" and the eval function.

Just a note that this bug still exists in PHP version 5.2.17.

I don't think this is bogus. Based on the quoted manual, the bug is that "execution of the following code continues normally" part does not happen when display_error is OFF, but happens when display_error is ON.