ruby_extend.rb

class A
  def a_method
    "a_method"
  end
  
  @@a_class_attribute = "A::a_class_attribute"
  def a_class_attribute
    @@a_class_attribute
  end
  
  def self.a_class_attribute=(value)
    @@a_class_attribute = value
  end
end

a = A.new
a.a_method # => "a_method"
a.a_class_attribute # => "A::a_class_attribute"

module B
  def a_method
    "b_method"
  end
  
  def self.extended(object)
    super
    
    # this modifies the class_attribute of the A class
    object.class.a_class_attribute = "B::a_class_attribute"    
  end
end

b = A.new
b.extend B
b.a_method # => "b_method"
b.a_class_attribute # => "B::a_class_attribute"

a.a_method # => "a_method"
a.a_class_attribute # => "B::a_class_attribute" # probably not intended