To be filed under ‘Most irritating of Internet Explorer bugs’, we discovered today, eventually, that Internet Explorer has issues dealing with cookies sent from Codeigniter and Lussumo’s Vanilla forum software if the sub-domain it is hosted on contains an underscore. I can't express how much time was wasted on this, or how many avenues were explored, (custom session libraries, URL based session identifiers ...), before the answer was unexpectedly found. This is a reminder and an attempt to get the resolution out there for the next poor soul who has this problem.