This is a question that I am going to answer myself, since I have seen it countless times, and this really should be considered a FAQ.
Suppose you have created a flash drive or a Django application, and it works both in your test applications - outside bluemix and inside bluemix. Then you make changes or enable a new pips import, and now your application still works in your local environment, but it does not work when you click it on Bluemix.
You can see something like:
OUT Starting app instance (index 0) with guid ....
ERR Instance (index 0) failed to start accepting connections
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