Not unique to Drupal, but common to any that use cookies to maintain login session information. I’ve met it before, but I just lost an hour and I have a brick shaped imprint in my forehead again, so I thought I’d drum into my head … don’t use underscores in domain names!

So often when using subdomains for setting up dev sites etc., it’s so easy to use site_dev.domain.com or similar. But, underscores are not valid characters in domain names.

Firefox is forgiving, and behaves as you would hope, but IE7, Opera etc. won’t properly set cookies on domains with underscores.

Solution – use hyphens instead e.g. site-dev.domain.com