How to reproduce:

require 'rubygems'
require 'mongoid'

Mongoid.configure do |config|
config.master = Mongo::Connection.new.db("mongoid-history")
end

class Post
include Mongoid::Document

field :title
field :body

embeds_many :comments
end

class Comment
include Mongoid::Document

field :title
field :body

embedded_in :post, :inverse_of => :comments
end

post = Post.create(:title => "test", :body => "test")
comment = post.comments.create(:title => "test", :body => "test")
comment.reload

Output:

Mongoid::Errors::InvalidCollection: Access to the collection for Comment is not allowed since it is an embedded document, please access a collection from the root document.
from /Users/aqian/.rvm/gems/ruby-1.9.2-p180@moe/gems/mongoid-2.0.0.rc.7/lib/mongoid/collections.rb:21:in `collection'
from /Users/aqian/.rvm/gems/ruby-1.9.2-p180@moe/gems/mongoid-2.0.0.rc.7/lib/mongoid/collections.rb:11:in `collection'
from /Users/aqian/.rvm/gems/ruby-1.9.2-p180@moe/gems/mongoid-2.0.0.rc.7/lib/mongoid/document.rb:145:in `reload'
from (irb):28
from /Users/aqian/.rvm/rubies/ruby-1.9.2-p180/bin/irb:16:in `<main>'