Facebook iFrame App - Infinite loop after auth

Note the trailing slash

So, you've followed all the tutorials you can find in the world and after some extreme hair-pulling, things still don't work

Note
i'm using the PHP SDK and the auth flow example from github

The Problem
After authentication, you are thrown into an infinite loop:
iframe canvas app > fb login.php > yoursite redirect url > and repeat

The Solution
go to your App Setting > Facebook Integration > Page Tabs heading, and make sure your Tab URL has a trailing slash on it.

This may not solve your problem, but it's worth checking.