It happens on my Mac when the window width hits approx 760px. It looks like your collapse-o-script is failing on the forum pages, for some reason. (Also, odd that you omit a 2000 AD logo from the nav strip on the narrow site.)
Okay, I think I see what you mean now, yes that doesn't look right - we'll look into this. It also doesn't seem to be collapsing the menu on an iPhone, but it's meant to!
It should look more like the www.2000adonline.com
site does when the menu gets collapsed for small screens, though won't be exactly the same as the forum doesn't use Foundation.. On the www.2000adonline.com
site it looks like this could still do with some further tweaks, though, so that the 2000 AD logo is fixed top left, rather than being moved to the burger navigation drop-down. We'll see what we can do.