There's some goofy behaviour I've noticed in Google Chrome. Here's how to reproduce it.
Put your cursor somewhere in this region
Or anywhere in the site's header where you can middle-click, really. Then do that: middle click to get that scrolling cursor happening, and move your cursor to scroll left, right, or up.
This happens! Whoooaaa!
Interestingly, you can scroll until the point you middle-clicked on leaves the header div.
Some fix options
Any of these seem to work.
Remove #content's overflow: auto
Remove #subheader's overflow-y: hidden
Add an overflow-x: hidden to #subheader