Power Supply Failure
If you notice the lights flickering or the machine randomly shutting down, this is probably a failure in the power supply. In some cases the machine may even randomly restart. The power supply can be shaken loose because the machines are bumped so often by the user.
Flippers Stick
Because the flippers get used so often, they will become jammed or stuck from time to time. Shorts in the electrical systems can also effect the reaction time of the flippers. The problem can also be caused by a physical malfunction if the flippers aren't oiled often enough.
Ball Shooter Malfunction
Pinball machines with spring operated ball shooters will occasionally get stuck if the spring is old and/or rusted. This will change the amount of power that can be used to put the ball into the play area. In newer machines, the ball shooters are often operated by simply pushing a button. These ball shooters can also have problems working at all if there is a malfunction in the electrical system.
Optical Sensors
When the pinball is shot into special score areas, certain games will have a physical function that gets triggered. For example, in the Addams Family pinball game, the character Thing will pick up a ball and carry it into another area that gives you a bonus score. These functions occasionally stop working due to a failure in the optical sensor that triggers them.
Digital Screens
Because the digital screens consist of LED lights, it is very easy for individual lights to burn out over time. If the pinball machine experiences electrical issues, the entire screen may fail to work or shut off all at once.