The following type of HTML in the bookmarks causes an underflow in @category_stack:
<DT><H3 LAST_MODIFIED="1207834166" PERSONAL_TOOLBAR_FOLDER="true" ID="rdf:#$StgZz3"></H3>
Because there is no text between the H3 tags, the "text" callback in Bookmarks.pm is never called, and @category_stack proceeds to underflow when "end" is called when it reaches a </DL>.
This kind of "empty" H3 occurs in the "Bookmarks Toolbar Folder" it seems, so this bug is likely to strike a lot of people.