This is an eigenclass (singleton class) annoying bug that was fixed in Ruby 1.9.2

eigenclass_bug.rb

# Add method filter to array

class Array
  
  def filter(str)
    
    self.sort.select do |m|
      m =~ /^#{str}/i
    end
  end

end

class A 
  
  def m_a
    puts "inside m_a"
  end
  
end

class B < A
  
  def m_b
    puts "inside m_b"
  end
  
end

puts "Test 1:"
p B.instance_methods.filter('m_') # both 'm_a' and 'm_b' are listed
p B.instance_methods(false).filter('m_') # superclasses are ignored 
                                                   # only 'm_b' is listed
# ok, now let's move to the eigenclass

class Object
  
  def eigenclass
    class << self
      self
    end
  end
  
end

b1 = B.new

def b1.m_only_from_b1
  puts "only from b1"
end

puts "Test 2:"
p A.instance_methods.filter('m_') # m_a is listed
p B.instance_methods.filter('m_') # m_a and m_b are listed
p B.instance_methods(false).filter('m_') # only m_b is listed

puts "Test 3:"
p b1.eigenclass.instance_methods.filter('m_')

# THE RUBY 1.8.7 BUG:
p b1.eigenclass.instance_methods(false).filter('m_')

## Ahaaaa! This was fixed in ruby 1.9.2. It was buggy on ruby 1.8.7
# In Ruby 1.8.7 the output was: ["m_b", "m_only_from_b1"], in other words, "m_b" was incorrectly returned
# In Ruby 1.9.2 the output is now: ["m_only_from_b1"] which is what one would expect

# Thanks for the Ruby team for correcting that annoying bug...