class Parent
  include Mongoid::Document
  embeds_one :child, :cascade_callbacks => true
end

class Child
  include Mongoid::Document
  before_create :always_fail
  def always_fail
      puts "failing..."
      errors.add :base, "boo hoo"
      false
   end
end

1.9.3p194 :041 > p = Parent.new
 => #<Parent _id: 4ffb17ce73f05750ce000005, _type: nil> 
1.9.3p194 :042 > p.build_child
 => #<Child _id: 4ffb17de73f05750ce000006, _type: nil> 
1.9.3p194 :043 > p.save
failing...
 => true 
1.9.3p194 :044 > p.child.errors
 => #<ActiveModel::Errors:0x00000005ae7d10 @base=#<Child _id: 4ffb17de73f05750ce000006, _type: nil>, @messages={:base=>["boo hoo"]}> 
1.9.3p194 :045 >  p.child.persisted?
 => true 
1.9.3p194 :046 > p.child.changed?
 => false