If you rename a post you can break urls pointing to your blog. If you set the "old-slug" field on the front matter to the old permalink, this plugin will create a symlink between them.
---
title: Hello world
author: me
old-slug: /2013/08/25/hello-world.html
---
Hello!
For example, if you change permalink from "pretty" to ":title.html", this plugin will write the post to "hello-world.html" and create a symlink to it on "/2013/08/25/hello-world/index.html" (the "pretty" version), so both urls will work.