I used in the project slightly modifying class UrlFetchRpcHandler (file /appengine/standard/urlfetch/async/rpc.py)
Approximately once every 100,000 requests to receive an unexpected error:

UnboundLocalError: local variable 'result' referenced before assignment

It looks like the problem of garbage collection. Completely solve the problem by adding at the beginning define of the line

global result

I hope it will be useful.