See http://redmine.ruby-lang.org/issues/3150#change-16042

gistfile1.rb

require 'net/http'
require 'net/https'
require 'openssl'

url = URI.parse("https://github.com/")       # OK
#url = URI.parse("https://etutorials.org/")  # Hostname mismatch

http = Net::HTTP.new(url.host, url.port)
http.use_ssl = true
http.verify_mode = OpenSSL::SSL::VERIFY_PEER

http.verify_callback = proc do |preverify_ok, ssl_context|
  puts "preverify_ok: #{preverify_ok}"
  puts "error: #{ssl_context.error}"
  preverify_ok
end

request = Net::HTTP::Get.new(url.path)
response = http.request(request)   # Should raise error