It’s worth noting (since this shows up in the search engines, which is how I found it) that this is deprecated. The Rails 3 way of doing it is below but is no longer valid as of Rails 7 (see the comment from April 2022)
errors[:base] << "Error message"
the preferred way of doing it is
errors.add(:base, "Error message")
http://apidock.com/rails/ActiveRecord/Errors/add_to_base
http://apidock.com/rails/v3.2.8/ActiveModel/Errors/add