checkout the following issues: http://code.google.com/p/googleappengine/issues/detail?id=1758&q=model%20exception&colspec=ID%20Type%20Status%20Priority%20Stars

checkout the following issues: http://code.google.com/p/googleappengine/issues/detail?id=1758&q=model%20exception&colspec=ID%20Type%20Status%20Priority%20Stars%20Owner%20Summary%20Log%20Component%20Language http://code.google.com/p/googleappengine/issues/detail?id=17... http://code.google.com/p/googleappengine/issues/detail?id=1409 http://code.google.com/p/googleappengine/issues/detail?id=14... you need to trap DeadlineExceededError for long-running queries, CapabilityDisabledError (from google.appengine.runtime.apiproxy_errors) for when the datastore is in maintenance mode and other misc errors. If it is a deadline error, you can re-submit it (but prob advisable to only re-sub once), on CapabilityDisabledError, return a server error page or load from memcache