Part of Python [removed] core module on App Engine has a bug in it. Including this module will terminate your application with an exception.
Nobody notices it in development or testing, because the dev system was including the python distributions [removed] module, which is apparently different to the one on App Engine.
Site makes it through testing. push live and crunch, falls over.
Now we have a test app engine account, and have to deploy to that first, because we cant rely on the installation of Python on App Engine matching the installation of python on our dev machines, even though the versions are exactly the same
That's good practice. You test locally, then test on a real deployment environment, then deploy to production. You should also use app versions and test new ideas on new versions and always plan for rolling back.