It has to do with how the optical flow sensor works. "These are camera modules that use ground texture and visible features to determine aircraft ground velocity." In order to do this, the cameras view must have contrast to it. If the camera view doesnt have any contrast the sensor will assume that it is faulty. You can see this error if you download the flight log. Near the end of the text file, you should see a line similar to this "Problem with optical flow, there has been no flow for 50 cycles in cycle 650: shutting down motors now".
Take a look at this video to see the proof ( https://www.youtube.com/watch?v=aUkDc4D4d4g ). The easiest way to fix the issue (according to Matlab Tech Support) is to add some contrast under the Parrot, and ensure that the area is well lit.
-Brandon
