Right now it happens a lot for our Mozmill CI project that we want to download a build which has been finished right at the same time. Sadly we get back a 404 when trying to do so. Means we probably end-up in getting cached content returned and the new build hasn't been indexed yet.
So we should do similar like this bug with Cache-Control: max-age=0 or by adding a random query string at the end of the URL: http://ftp.mozilla.org/.../firefox-15.0a1-win32.zip?r=12345. See bug 757122.
Disable caching when fetching build information (#13)
Pull has been merged.