diff --git a/actionpack/lib/action_controller/dispatcher.rb b/actionpack/lib/action_controller/dispatcher.rb
index 781bc48887..9374a7f060 100644
--- a/actionpack/lib/action_controller/dispatcher.rb
+++ b/actionpack/lib/action_controller/dispatcher.rb
@@ -7,7 +7,6 @@ module ActionController
         unless cache_classes
           # Development mode callbacks
           before_dispatch :reload_application
-          after_dispatch :cleanup_application
 
           ActionView::Helpers::AssetTagHelper.cache_asset_timestamps = false
         end
@@ -93,11 +92,9 @@ module ActionController
       run_callbacks :prepare_dispatch
 
       Routing::Routes.reload
-    end
 
-    # Cleanup the application by clearing out loaded classes so they can
-    # be reloaded on the next request without restarting the server.
-    def cleanup_application
+      # Cleanup the application by clearing out loaded classes so they can
+      # be reloaded on the next request without restarting the server.
       ActiveRecord::Base.reset_subclasses if defined?(ActiveRecord)
       ActiveSupport::Dependencies.clear
       ActiveRecord::Base.clear_reloadable_connections! if defined?(ActiveRecord)
diff --git a/actionpack/lib/action_controller/session/abstract_store.rb b/actionpack/lib/action_controller/session/abstract_store.rb
index 9434c2e05e..41a35f867f 100644
--- a/actionpack/lib/action_controller/session/abstract_store.rb
+++ b/actionpack/lib/action_controller/session/abstract_store.rb
@@ -58,9 +58,28 @@ module ActionController
           end
 
           def load!
-            @id, session = @by.send(:load_session, @env)
-            replace(session)
-            @loaded = true
+            stale_session_check! do
+              @id, session = @by.send(:load_session, @env)
+              replace(session)
+              @loaded = true
+            end
+          end
+
+          def stale_session_check!
+            yield
+          rescue ArgumentError => argument_error
+            if argument_error.message =~ %r{undefined class/module ([\w:]*\w)}
+              begin
+                # Note that the regexp does not allow $1 to end with a ':'
+                $1.constantize
+              rescue LoadError, NameError => const_error
+                raise ActionController::SessionRestoreError, "Session contains objects whose class definition isn\\'t available.\nRemember to require the classes for all objects kept in the session.\n(Original exception: \#{const_error.message} [\#{const_error.class}])\n"
+              end
+
+              retry
+            else
+              raise
+            end
           end
       end