Static discharged through a penny. Most people don't actually see static discharge, they just feel the jolt, they feel it because it's jumping through their skin, you won't feel the shock if you let it jump from a piece of metal like a coin thus creating a spark. I was able to build enough static energy just by simply taking off a fleece sweater, and it had enough energy for a second spark after the initial one. On a technical aspect, this is why you should always wear an anti-static wrist strap before working on any electronics, also why you should never enter a confined space that contains gas fumes before it's vented properly.
