Gemfile.rb

gem 'rake'

group :never do
  gem 'json'
end

source 'http://gemcutter.org'

output.txt

$ bundle install
Fetching source index from http://gemcutter.org
Resolving dependencies
Installing json (1.2.0) from system gems 
Installing rake (0.8.7) from system gems 
Your bundle is complete!
bundle install  11.14s user 0.20s system 84% cpu 13.477 total

$ bundle exec rake
(in /Users/adelcambre/p/playpen/bundle_fails/require)
[]
rake aborted!
Don't know how to build task 'default'

(See full trace by running task with --trace)

$ bundle lock 
The bundle is now locked. Use `bundle show` to list the gems in the environment.

$ bundle exec rake
(in /Users/adelcambre/p/playpen/bundle_fails/require)
["json/version.rb",
 "json/common.rb",
 "json/ext/parser.bundle",
 "json/ext/generator.bundle",
 "json/ext.rb",
 "json.rb"]
rake aborted!
Don't know how to build task 'default'

(See full trace by running task with --trace)

Rakefile.rb

Bundler.require :default

require 'pp'

pp $LOADED_FEATURES.grep(/json/)