Two days ago I crashed the quadcopter very hard. I flew in a steel pillar and the copter fell down about 3 meters on a stone stair. Both center plates are destroyed and three of my well balanced props are damaged. The battery also got a very small dent. I'm not sure if it got damaged seriously but I think I will replace it, cost of such a battery pack is just 14$ on HK. Luckily the motors and the other parts are all ok.
The replacement parts were milled very fast. I also decided to replace the cam support. The new one has four rubber balls because my old design was a bit unbalanced. The copter of my classmate, which we built some weeks ago, is already using these parts. By the way, he is using a GoPro. I may also buy such a cam in the future. The quality is so much better compared to my HK wingcam.
To improve stability, program all your ESC's the same way. I never did this because I didn't had any problems until we built the copter of my classmate. I may try to flash the ESC's with a SimonK firmware sometimes in the future, but currently I'm working on my new CNC router and the new semester will start on Monday. So no time for this at the moment.