I was fooling around on the forum, minding my own business, when a pop-up appeared. It said something along the lines of (I don't have exact wording) 'The site
www.heresy-online.com is asking you to log in. It says "Under Maintenance".' After this there was a space for a username and password. I escape-keyed out of it and didn't give it a second thought until a friend mentioned the same thing happening, at which point I thought I should bring it to the attention of the board staff.
If it helps, I run Ubuntu and my friend runs Windows 7, so I strongly suspect that whatever it is is forum-based.