I am trying to test the failure paths in my Go GAE application. I am testing using the included appengine/aetest package that creates a dev_appserver.py subprocess to handle API calls, however it seems impossible to tell the API stub to start failing (it always works).
A promising thing I tried was using withmock to actually mock out the memcache package and make the functions return the series of errors I'd like. Alas I found withmock is not very compatible with App Engine.
This sounds like a good feature request. Please file one over on the App Engine issue tracker!