Straight from the PinWiki.....

4.10.9 Switch Capacitor Purpose and Failure

Capacitors are used on switches that are normally subjected to very quick hits, to lengthen the switch pulse that the MPU reads. Not all switches will have capacitors; standups, pop bumpers, slings, and tilt mechanisms are the usual ones that benefit from them. All switches that need a quick reaction can have switch capacitors added to them (such as rollover buttons). These are ceramic non-polarized capacitors of a value of .047 or .050 uf and 16 volts or greater. A commonly available cap is the .047 uf 50 volt through Mouser P/N 140-50U5-473M-RC.

The factory installed caps are often damaged by the high wattage industrial soldering irons used in manufacturing the games, or they succumb to the vibration over time and no longer serve their purpose of lengthening the switch closures so the MPU can read them. Often a repairman will clip the cap off to diagnose a stuck switch problem and not bother to replace it. The switch will operate in this fashion, but for more sensitive play, the capacitor should be replaced. A failing cap can also make a switch bounce, and cause machine-gunning of a slingshot or pop bumper.

Sometimes the operation of the flippers or other solenoids will cause a phantom switch close on a pop bumper or slingshot - this is due to the way the software reads the fast react switches on those mechanisms. The RF produced by the flipper coil's collapsing EMF field is picked up by the switch capacitor, causing the pop or sling to fire. In most cases, no score is added, the mechanism just operates. Keeping the leads short on the switch capacitors might reduce their tendency to pick up this stray RF in the machine.