Pages with unexpected page layout (contentlayout) structure no longer have some content rendered

It turns out that some customers have pages that contain multiple contentLayout divs. Since the OnDemand upgrade from Confluence 5.2-OD-5 to 5.2-OD-13, these pages no longer behave like they used to and are missing content when rendered (either for viewing or editing).

Inspecting the document storage format, the content is all still there. It might look like this, where only the content in the final contentLayout div is rendered:

<div class="contentLayout" ...>
  (content that is NOT rendered)
</div>
<div class="contentLayout" ...>
 (content that is rendered)
</div>

or this, where nothing on the page is rendered:

<div class="contentLayout" ...>
  <div class="contentLayout" ...>
    <div class="innerCell">
      (content is here but not rendered)

These duplicate contentLayout divs should be cleaned up by the editor on the fly so that the content will be displayed. OnDemand customers have no way to edit the storage format data directly on their own.