In Ruby 1.8 sending :attr_accessor to an object resulted in a public accessor method. In Ruby 1.9 the methods are private.

priv_attr_accessors.rb

class Foo
  attr_accessor :x
end
Foo.send(:attr_accessor, :y)

myfoo = Foo.new
# Using the attr_accessor defined initially works fine
myfoo.x = 1

# Using the attr_accessor defined at runtime gies NoMethodError
myfoo.y = 1
NoMethodError: private method `y=' called for #<Foo:0x000001008f6be0 @x=1>
        from (irb):9
        from /Users/bklang/.rvm/rubies/ruby-1.9.2-p0/bin/irb:17:in `<main>'