output

➜ ruby --version
ruby 1.9.3p194 (2012-04-20 revision 35410) [x86_64-darwin11.4.0]
➜ ruby respond_to_missing.rb 
Using #method works: 
foo_bar
But #method_defined? doesn't:
method defined: false
And Module#instance_method raises a NameError:
respond_to_missing.rb:20:in `instance_method': undefined method `foo_bar' for class `Foo' (NameError)
	from respond_to_missing.rb:20:in `<main>'

respond_to_missing.rb

class Foo
  def method_missing(name, *args)
    return super unless name =~ /^foo_/
    puts name
  end

  def respond_to_missing?(name, include_private)
    super || name =~ /^foo_/
  end
end

puts "Using #method works: "
Foo.new.method(:foo_bar).call

puts "But #method_defined? doesn't:"
puts "method defined: #{Foo.method_defined?(:foo_bar)}"

puts "And Module#instance_method raises a NameError:"
Foo.instance_method(:foo_bar)