So I was editing a bunch of photos I took whilst on holiday. My left arm remained static hovering over a few keys with my thumb on my iMacs metal keyboard whilst my left elbow just touches the Macbook. It took a while (and reproducible!) but after 10 seconds it gets too much and feels like something building up. I don't want to leave my arm there long enough to get a proper shock off it . Whenever my unibody Macbook or older 12" PB are plugged into the mains running your finger over it quickly makes it feel bumpy too. It's not just our mains either! Happened at other homes, universities etc. Anyone else done this?