What happens in machining, dies wear down, crack an break after hundreds of thousand whacks. The devices (letters/numbers) become wider, less distinct. The other factor is machine vibration and heat build up, all takes a toll. The thin plating of copper reacts to these effects. An error is that which is outside of the normal stamping process. Read up on what is the cause of doubled dies, and how RPM's are introduced when the dies are made.