Drupal and cookies domain caching

From time to time anyone experience some problems with non-fresh Drupal installations (e.g. some project from SCM). The major one for me was some strange cookie setting and handling - when I tried to login Drupal replied with correct Set-Cookie header, but cookies were not saved within browser, i.e. after login the page was just reloaded with login prompt. No message or anything to help me. So I just started commenting and editing related code by random. Finally, I got it. The problem is that Drupal was installed first with hostname A and I tried to work with it on hostname B. Don't know why sometimes it's okay and sometimes it's not. However, you just need to uncomment $cookie_domain and set it to your hostname in settings.php. Just like this:

$cookie_domain = 'your.domain';