based on your report that you haven't changed anything in the templates, I focussed on the posts' content, and in this case I used Firefox' web developer add-on and checked the CSS with 'Display Style Information' to see if the 'normal' structure of the post divs was interrupted;Could you tell me how you knew it was that article?
also, you could clearly see that, on the front page, the width of the posts column was getting disturbed near the bottom of that particular post.
is all down to a bit of 'deja-vu' ...