Remember folks, for apps that use the back-end as a JSON service, nearly all the code is running on the client. If you have no feedback about errors, you are assuming your 15 minutes of testing with Safari on a Macbook is representative of the entire Internet including that guy with IE7 on XP with the Bing Toolbar. It's not a good bet.
They mention tinyfeedback but there is also DamnIT and the YC-funded proxino.