Thinking about it a little more, some of the changes in software versions had also been to attempt to remove html from language file defines. Depending on how the latest upgrade was performed it could be possible that some product_info language define desired to be kept also had html in it. Trust me I'm shooting spitballs here. I agree, using an html validator would make the inspection easier. Might suggest also inserting some inane html code at desired locations to be able to track the impact of review.

Part of me also was wondering about the possibility of some sort of imbalanced php closure such that some tag or another is unfortunately outside of a grouping. Again, spitballing.