WampServer

put this script in a web accessible page, and then hit the page multiple times

you'll find that when two different web requests are running exec at the same time, apache freezes

<?php

for($i = 0; $i < 25; $i++)
{
    print($i);
    exec("dir";
}

?>

nevermind, i'm an idiot, see [bugs.php.net] and [us2.php.net]