Why is err_header_out needed to set cookies when redirecting?

[Sam Tregar]

Hello all.  Check out this bug:

     my $r = Apache->request;
     $r->header_out(Set-Cookie => '...cookie here...' );
     $r->header_out(Location   => '...url here...');
     return REDIRECT;

Now, we all know that second line has to be err_header_out not
header_out, or Apache will silently ignore the Set-Cookie directive.
I knew it as soon as I saw it, although I couldn't explain why and
thus didn't trust my intuition.  I looked through the list archives
and the guide without finding an explaination that made sense.

Would anyone mind explaining this to me?

Thanks,
-sam

PS: This is Apache v1.3.33 with mod_perl v1.29, in case it matters.