Caps and rotors get dirty, especially with carbon worn off the contact point in the cap. This carbon eventually gets widespread enough to create a current path to short out the spark and send it to ground through the distributor body rather than the spark plugs. It can happen inside the cap or on the rotor; in the former case, the carbon tracks are often visible and mistaken for "cracks" in the cap. Sometimes only one cylinder is shorted out in the cap, but more often the carbon track runs from the center electrode out to one of the plug wire electrodes and causes all sparks to go to that one cylinder, making the car impossible to start, but producing spectacular backfires when the wrong cylinder fires with the exhaust valve open and the exhaust system full of unburned gas.