Ruby undef vs. remove_method

wacky_undef.rb

class A
  def hello
    "hello from A"
  end
end

class B
  def hello
    "hello from B"
  end
end

module Mixer
  def hello
    "hello from Mixer"
  end
end

A.class_eval do
  undef hello
  include Mixer
end

B.class_eval do
  remove_method :hello
  include Mixer
end

A.new.hello #=> NoMethodError: undefined method "hello" -- undef banished this method from A forever?!?!
B.new.hello #=> "hello from Mixer"