Near the end of the mentioned chapter at https://www.freebsd.org/doc/en/books/porters-handbook/port-upgrading.html is sentence "Send the patch following the guidelines in Chapter 10, Upgrading a Port.", where text beggining Chapter 10... links to page self. Author probably meant to link to chapter 4. - Writing the Problem Report?
Mmmm, no, it is referencing the bit just before section 10.1, the paragraph before last, that starts with "To submit the diff". It should, maybe, be rewritten/clarified.
A commit references this bug: Author: jgh Date: Mon Jul 27 21:38:06 UTC 2015 New revision: 47120 URL: https://svnweb.freebsd.org/changeset/doc/47120 Log: - address chapter 10 improperly linking to itself PR: 193698 Reported by: knezour@weboutsourcing.cz Approved by: wblock (mentor) Differential Revision: https://reviews.freebsd.org/D3173 Changes: head/en_US.ISO8859-1/books/porters-handbook/upgrading/chapter.xml