Left Nav Bar Drops Below Content - but only for one taxonomy term

I don't even know where to start looking for this one, so I am hoping that someone can point me in the right direction.

I have a few category terms set up at the same level in the taxonomy heirarchy.

Authors (term 4)
- Author #1 (term 39)
- Author #2 (term 40)
- Author #3 (term 50)

When I do taxonomy/term/39 or taxonomy/term/50 - the page draws correctly with a nav bar on the left and the content window on the right.

When I do taxonomy/term/40 - the left nav bar drops below the content section.

I have looked at the html source for 39 & 40 at the point where it transitions past the node teasers, the pager and into the sidebar-left div and can see no differences. There are no differences in the way the terms are defined.

I saw a reference to a similar problem related to DHTML in http://drupal.org/node/361888 - but the DHTML module is not installed here.

I am using Taxonomy Access, but none of these terms have been touched there.

Any idea what may be happening here?

Thanks in advance.