I just ran into a scenario where I implemented a Remember Me checkbox on a login page (powered by CodeIgniter).  Turns out that if you use CodeIgniter Cookies and you have an underscore in the cookie name, that IE9 has a fit and won’t let you login.

So my login controller had something like this in it…

$this->load->helper('cookie');
$this->data['remember_email'] = '';
if (get_cookie('site_remember') != '') {
$this->data['remember_email'] = get_cookie('site_remember');
}

The simple fix was to remove the underscores from the cookie name and it worked great.