Something that caught be out week was the introduction of the secret_key base in Rails 4.

When adding a new environment make sure to add the secret_key_base for your environment in config/secrets.yml.

  
    development:
      secret_key_base: YOUR_DEVELOPMENT_KEY_HERE
    test:
      secret_key_base: YOUR_TEST_KEY_HERE
    staging:
      secret_key_base: YOUR_STAGING_KEY_HERE
    production:
      secret_key_base: YOUR_PRODUCTION_KEY_HERE