I haven't had the time to check if this is particular to being embedded. I'm using ruby 2.0.0-p0 and mongoid 3.1.3, I previously had mongoid 3.0.19 which also displayed unintended behavior.

The relevant part of my model is:

class Foo
  include Mongoid::Document
  field :lorem, type: Date

  embedded_in :bar
end

class Bar
  include Mongoid::Document
  include Mongoid::Timestamps

  belongs_to :user

  embeds_many :foo

  accepts_nested_attribuets_for :foo

  validates_presence_of :user
end

The problem I have is when updating attributes on a controller on rails:

@bar = current_user.bar
# given that params[:bar] contains a Foo with and invalid :lorem, such as "2013-04-00"
# when it's valid it works ok, but when not, this very next line will cause an exception
if @bar.update_attributes(params[:bar])
  # ...
else
  # ...
end

On mongoid 3.0.19 that exception is ArgumentError: argument out of range.
On mongoid 3.1.3 that exception is

  undefined method `__bson_dump__'

.

I've looked up for the latter and found out that mongoid won't work properly if bson_ext is loaded, however there are no bson gems whatsoever on my Gemfile.lock, thus it's probably not the case.