The upgrade instructions for 2.1.2 to 2.1.3 say to:
replace your index.php file. If any modifications were made to your index.php they will need to be made fresh in this new one.
Not the biggest deal, but since I use custom index.php files to run multiple CI sites from one CI framework, this would be several steps for me.
However, the index.php file included in these two versions is identical. So, in fact, this is an unncessary step they are telling you to take.