gistfile1.rb

CONSTANT = "in <global>"

class Foo
  CONSTANT = "in foo"
end

class Foo::Bar
  puts CONSTANT # => "in <global>"
end

class Foo
  class Bar
    puts CONSTANT # => "in foo"
  end
end

This quite often bites when developing namespaced engines for Rails, e.g:

class Blog::Post < ActiveRecord::Base
  def self.recent
    order("created_at DESC").limit(5)
  end
end

class Blog::PostsController < Blog::ApplicationController
  def index
    # this fails because it's looking for Post in global scope
    @posts = Post.recent
  end
end

module Blog
  class PostsController < ApplicationController
    def index
      # this works as it's looking for Post in Blog scope
      @posts = Post.recent
    end
  end
end

It can get especially tricky when in development mode and constant autoloading is in effect - you end having to use require_dependency to make sure that the constant you need is loaded properly.