Using ActiveModel::Validations standalone.

validations.rb

# What I should have to do:
require 'active_model/validations'

# What I actually have to do:
require 'active_support/callbacks'
require 'active_support/concern'
require 'active_support/core_ext/module/delegation'
require 'active_model/callbacks'
require 'active_model/errors'
require 'active_model/naming'
require 'active_model/translation'
require 'active_model/validator'
require 'active_model/validations'

The dependency on active_model/errors isn't even apparent until runtime when #valid? is called.