To reproduce bug do this test:
1) Turn off cookies at browser.
2) run file:
<?
session_name ("SESSID");
session_set_cookie_params (3600); 
session_register("test"); //registring tracks variable. 
if (!isset($test)) $test=1;
$test=$test++;
echo '<a href="'.$PHP_SELF.'?SESSID='.session_id().'">test</a>';
echo '<br><a href="'.$PHP_SELF.'?SESSID=c:\123">test2</a>';
?>
3) Click on a "test2" link
4) Apache child will crash and php will write message like
Warning: Failed to write session data (files). Please verify that the current setting of session.save_path is correct (/tmp) in Unknown on line 0

Testes on OS/2 (PHP 4.1.2) and win32 (PHP 4.1.1). PHP is installed as Apache module.