I have seen flutter only once in person. It was with a really overpowered Great Planes Fundango. It had an E-Flite Park 480 with a 3s battery. This made the plane travel way too fast and it fluttered violently where the pilot chopped throttle and it actually came down in one peice. One of the CA hinges had broken off of the aileron, but that was about the extent of the damage. It was incredible how violent flutter is, especially when you can see it happening in the air!