A couple of suggestions.
* Does the test fail if you run it alone from the command line (outside of rake)
$ cd my_app
$ ruby test/functional/alerts_controller_test
* Output the http response in the body of the test and see what is
actually getting returned.
puts @response.body # (maybe this is response.body?)
* Are you rendering any partials within the index template? There is
(or at least was) a nasty bug in the compiling of partials which can
lead to them working only in a certain order.