As far as I noticed that before_save callbacks raise Errors::Callback until I use 'errors.add'. 'errors.add' makes it throw Errors::Validations when the callback method returns false.

However, there is a situation when this is not true. I am not sure whether this is an issue or not. Also sorry for the example. It is a simplified version of something not that crazy.

rails new silicone -O
cd silicone
echo "gem 'mongoid'" >> Gemfile
bundle
rails g mongoid:config
rails g scaffold post title:text
rails g scaffold comment content:text

nano -w app/models/comment.rb

class Comment
  include Mongoid::Document
  field :content, type: String

  belongs_to :post
  before_save :check_content

  # recommended (?)
  validates_presence_of :post
  # not recommended (?)
  #validates_presence_of :post_id

  def check_content
    if content == 'bad'
      errors.add :content, 'Bad content'
      return false
    end
  end
end

nano -w app/models/post.rb

class Post
  include Mongoid::Document
  field :title, type: String

  has_many :comments
end

rails console

p = Post.create; c = Comment.new; c.post = p; c.content = 'bad'; c.save!
Mongoid::Errors::Callback:
Problem:
  Calling save! on Comment resulted in a false return from a callback.

Note that:

Using 'validates_presence_of :post_id' instead of 'validates_presence_of :post' throws Errors::Validation. (What seems to be good.)
If you remove the 'has_many :comments' from post.rb then Errors::Validation is thrown instead of Errors::Callback.
Changing "errors.add :content, 'Bad content'" to "errors.add :base, 'Bad content'" doesn't help.
Normally errors.add in before_save callback throws Errors::Validation what I find a good and logical thing.

Mongoid::Errors::Callback is thrown when all of these are true:
You use 'belongs_to :something'
something.rb contains 'has_many :your_model'
'validates_presence_of :something' is used and not 'validates_presence_of :something_id' in your_model.rb
You have a before_save callback.
You return false in the callback but you also call errors.add before that.

I find it a little misleading when writing integration tests. It is not a big issue though. I noticed this when I wanted to standardize my errors and 'validates_presence_of' lines and removing some '_id'. I read that using '_id' in validates_presence_of is not recommended.