This shows how a singleton instance variable is shared across threads and can cause unexpected results

singleton_thread_safety.rb

# frozen_string_literal: true
require 'singleton'

class Fruit
  include Singleton

  def config
    @config ||= {}
  end

  def set(text)
    config[:text] = text
  end
end

t = Thread.new do
  Fruit.instance.set 'banana'
  puts 'text after first thread set (we expect banana)'
  puts Fruit.instance.config[:text]
  sleep 4
  puts 'text after first thread sleep (we expect banana, but it will be coconut)'
  puts Fruit.instance.config[:text]
end

Thread.new do
  sleep 2
  Fruit.instance.set 'coconut'
  puts 'text after second thread set (we expect coconut)'
  puts Fruit.instance.config[:text]
end

t.join