Do not allow to use plugin_new generator directly, you should use Usage:

rails new APP_PATH [options]

Options:
  -G, [--skip-git]            # Skip Git ignores and keeps
  -r, [--ruby=PATH]           # Path to the Ruby binary of your choice
                              # Default: /Users/drogus/.rvm/rubies/ruby-1.8.7-p302/bin/ruby
  -b, [--builder=BUILDER]     # Path to an application builder (can be a filesystem path or URL)
      [--edge]                # Setup the application with Gemfile pointing to Rails repository
      [--dev]                 # Setup the application with Gemfile pointing to your Rails checkout
      [--skip-gemfile]        # Don't create a Gemfile
  -d, [--database=DATABASE]   # Preconfigure for selected database (options: mysql/oracle/postgresql/sqlite3/frontbase/ibm_db)
                              # Default: sqlite3
  -O, [--skip-active-record]  # Skip Active Record files
  -m, [--template=TEMPLATE]   # Path to an application template (can be a filesystem path or URL)
  -J, [--skip-prototype]      # Skip Prototype files
  -T, [--skip-test-unit]      # Skip Test::Unit files

Runtime options:
  -s, [--skip]     # Skip files that already exist
  -p, [--pretend]  # Run but do not make any changes
  -f, [--force]    # Overwrite files that already exist
  -q, [--quiet]    # Supress status output

Rails options:
  -v, [--version]  # Show Rails version number and quit
  -h, [--help]     # Show this help message and quit

Description:
    The 'rails new' command creates a new Rails application with a default
    directory structure and configuration at the path you specify.

Example:
    rails new ~/Code/Ruby/weblog

    This generates a skeletal Rails installation in ~/Code/Ruby/weblog.
    See the README in the newly created application to get going. command

railties/lib/rails/commands.rb

@@ -14,6 +14,9 @@ case command
 when 'generate', 'destroy', 'plugin'
   if command == "plugin" && ARGV.first == "new"
     require "rails/commands/plugin_new"
+  elsif command == 'generate' && ARGV.first == "plugin_new"
+    puts "This generator should not be used directly as a generator. You should use `rails plugin new` command instead"
+    exit(1)
   else
     require APP_PATH
     Rails.application.require_environment!

railties/test/application/generators_test.rb

@@ -31,6 +31,12 @@ module ApplicationTests
       assert File.exist?(File.join(rails_root, "vendor/plugins/bukkits/test/dummy/config/application.rb"))
     end
 
+    test "don't allow running plugin_new generator as a generator" do
+      FileUtils.cd rails_root
+      output =  `./script/rails g plugin_new vendor/plugins/bukkits`
+      assert_match /This generator should not be used directly as a generator. You should use `rails plugin new` command instead/, output
+    end
+
     test "generators default values" do
       with_bare_config do |c|
         assert_equal(true, c.generators.colorize_logging)