I recently had a failure - fortunately on the ground.
I was using an "Alexmos" gimbal controller and APM 2.6 flight controller.
The gimbal controller was connected to the battery (16V) and to the radio. I bypassed the flight controller and used one channel of the radio to control the camera angle.
The insulation on the wire to the gimbal's accelerometer became worn and shorted to GND.
That started a cascade of problems:
The linear regulator (Input - > 5V) on the gimbal controller nearly instantly failed because of the short. The regulator itself shorted input -> output.
The shorted regulator fed 16V (battery voltage) directly to the 5V line. The shorted wire to the gimbal accelerometer instantly went up in smoke - since it now had 16V on it. That caused it to open.
The connection I had between the gimbal and the radio used 3 wires. (GND +5 and Signal).
The +5V wire from the gimbal now had 16V on it. It fed that voltage to the radio.
The radio was connected to the flight controller with 3 wires (GND +5, Signal). That fed 16V to the flight controller's 5V line.
The flight controller was connected to the GPS and the RFD 960 radio (all had GND and +5V connections).
Virtually everything in the system was powered by 5V. And most everything fried.
Moral of this story:
Cut the middle pin (5V) off the gimbal controller interface, so it can't feed voltage to the system. And then add a big 6V TransZorb across the flight controller + and GND pins to prevent high voltage from wrecking everything.