Error when creating a Matrix or installing a Plugin
Check to make sure you have a db.php file here:

craft/app/etc/config/defaults/db.php


If not, download a fresh copy of the version of Craft you're using and replace your craft/app folder with the one you just downloaded.

This file was missing in my Git repository because the .gitignore file was set to ignore all db.php files on the site, not just the one they wanted to ignore in the craft/config folder.

If that's the case, make sure your .gitignore file uses a line like this:

craft/config/db.php


Instead of just this:

db.php