... E.g. Multirotor with good compass calibration is on position hold in the air, Nose position is North, GPS data shows the unit moved to the South, controller will rev up the rear two rotors to correct the unit forward, unit moves forward back to it's original position, controller happy. Lets say the compass is now out of calibration it and says the nose is pointing north but in fact it is pointing more towards the West, again the GPS data says that the unit moved out of position to the South, controller thinks that the nose is pointing North therefore revs the rear two props up, since the nose is actually pointing west the unit will now move to the west, controller will see the GPS data out of position to the west thus will fire up the left props to move to the east, but in fact will move to the North,... the story goes on and on causing the unit to circle around the point it is supposed to be at.