If a document is modified in the staged site, publishing to live fails.
Steps to reproduce:
1. Enable local staging.
2. Add the 'Document and Media' portlet to the staged page.
3. Add a new document in it and publish to live. -> Publish successful.
4. On the staged page, change the title of the document and publish again to Live.
Result: The publishing process fails due to a NullPointerException. Please see the attached screenshot.