FlashHash override to handle both kinds of flashes i.e created in Rails 3.0.20 as well as in Rails 3.2.22. Put this config/initializers/. Kept with Rails 3.2.22 code.

flashhash_override.rb

# Supports both kind of flash messages (Rails 3.0.20 and 3.2.22)
# It converts Hash type of flash messages to Object type of flashes

ActiveRecord::SessionStore::Session.class_eval do

  def self.convert_to_3_2(flash_data)
    return unless flash_data

    flash_new = ActionDispatch::Flash::FlashHash.new

    used = flash_data.instance_variable_get('@used')
    flash_new.instance_variable_set('@used', used) if used
    flash_new.instance_variable_set('@flashes', Hash.new.merge(flash_data))

    flash_new
  end

  def self.unmarshal(data)
    return unless data

    marshalled = nil
    begin
      decoded = ::Base64.decode64 data
      marshalled = Marshal.load decoded
    rescue ArgumentError => ae
      if ae.message =~ /dump format error/
        with_alternate_flash_klass do
          marshalled = Marshal.load decoded
        end

        marshalled['flash'] = convert_to_3_2(marshalled['flash']) if marshalled.key?('flash')
      else
        raise ae
      end
    end

    marshalled
  end

  def self.alternate_flash_klass
    if ActionDispatch::Flash::FlashHash.ancestors.include? Hash
      Class.new() do # Rails 3.2.22 FlashHash
        def initialize
          @used    = Set.new
          @closed  = false
          @flashes = {}
          @now     = nil
        end
      end
    else
      Class.new(Hash) do # Rails 3.0.20 FlashHash
        def initialize
          super
          @used = Set.new
        end
      end
    end
  end

  def self.with_alternate_flash_klass
    temporary_flash_hash_klass = alternate_flash_klass

    original_flash_hash_klass = ActionDispatch::Flash::FlashHash
    ActionDispatch::Flash.send :remove_const, :FlashHash
    ActionDispatch::Flash.const_set 'FlashHash', temporary_flash_hash_klass

    yield

    ActionDispatch::Flash.send :remove_const, :FlashHash
    ActionDispatch::Flash.const_set 'FlashHash', original_flash_hash_klass
  end
end