I adopted most of this little script from user feedback in
the session_unset()'s annotations.  I ran into the same
problem, and think that it demonstrates it well.

<?php
function
dummy()
{
    global $a;
    session_unset();
    $a= 'foo';
    echo "<br>2. ".session_register('a');
}
session_start();
$a= 'foo';
session_register('a');
echo "<br>1. $a";
dummy();
echo "<br>3. $a";
?>

If you run it, you'll see that, even though the second 
session_register() returns TRUE, $a is not registered again,
and remains unset.

This bug does not allow me to be paranoid enough coding a
login-protected website.

Thanks.