The following is an error from Bundler, that can occur when a Gemfile refers to one version of a gem, but the gem has been upgraded to a newer version:

`check_for_activated_spec!': You have already activated i18n 1.8.5, but your Gemfile requires i18n 1.8.3. Prepending `bundle exec` to your command may solve this. (Gem::LoadError)

There are several options for handling the error:

  1. Follow the instructions, and prepend any commands with bundle exec (gemname)
  2. Upgrade the specific gem reference in Gemfile.lock, using bundle update (gemname)
  3. bundle update, but beware that this will update all gems referenced in a Gemfile without specific version numbers
  4. bundle clean, the most potentially destructive option, which will clean all the gems in your Ruby installation. This command must be run with the --force option, because it has the broadest effect.

Options 1 or 2 are probably the best bet when working in a professional environment, since the alternatives can affect other developers. (If working solo and unconcerned about version upgrades, options 3 and 4 may be fine.)