Oops, I broke Google?
Update: They're on it! :) In the meanwhile, the suggested workaround is to add debug=True to your template.render() methods (pass it as the third argument, i.e., after the context).
Update: As of 2.30pm (when I last checked), the app appears to be working again. I've made no changes whatsoever.
If you hit the Flash or Flex client on The GAE SWF Project right now, you will be greeted with a page long traceback.
I've just opened issue 273 on the Google App Engine Issue Tracker to track this and added this comment on the forums.
The problem is that the error never occurs locally when developing and testing with the Development Web Server and randomly starts happening on appspot (the app will work well for stretches of time and then start failing 100% of the time.)
The last time this happened, issuing an appcfg.py update . fixed the issue but I'm not going to do that now in hopes that seeing the error will make it easier for Google's engineers to fix it.