I encountered the following error when I try to create a new post in Octopress:

You have already activated rake 10.0.4, but your Gemfile requires rake 0.9.2.2. Using bundle exec may solve this.

This happened because I updated rake.

To solve, simple open Gemfile, and update the version number to the latest activated version on your system. eg.

gem 'rake', '~> 10.0.4'