When changing the lens, if dust or debris gets inside the camera and adheres to the surface of the image sensor (the part that converts the light to an electric signal), it may appear as dark spots on the image, depending on the shooting environment.
This camera is equipped with an anti-dust function that causes the image sensor to vibrate a little when the power is turned off to prevent dust from attaching to the image sensor. However, always make sure to attach/remove the lens quickly away from dusty locations.