Maybe you already know the diff between save and save!. "Save doesn't throw exceptions but its bang! version does when save failed."

Is it true?

Partly, only when u don't use locks. If you have a lock_version column in table, rails will enable lock automatically and in this case, ActiveRecord#save may throw StaleObjectError.

class AddLockToUser < ActiveRecord:Migration
  def self.up
    add_column :users, :lock_version, :integer, :default => 0
  end
#......
end

# somewhere in your controller
user1 = User.find 1
user2 = User.find 2

user1.boss = 'Bill Gates'
user2.boss = 'Rex'

user1.save
user2.save # here a StaleObjectError will be throw