I have a test which was failing with a timeout (waiting for a page element) on ec2, so I did some changes to it and ran it on a loop (-N 10) to make sure it wouldn't timeout again. Much to my surprise it failed every other iteration. After running locally with -D I realized it was failing because the second iteration of the test would start on the page where the previous one left off. That means the ensure_login() call would operate on a stale page and think the user was already logged in, but when you reload the page (or open a new one), the user isn't actually logged in (thanks to the new WindmillTestClient, created in the setUp() method, I guess).
This could easily be fixed by making ensure_login() always reload the page before doing anything.