It's happened a couple times now, randomly. As recently as earlier tonight. My watch was at 90% and all the sudden it wouldn't turn on when I tapped the screen. I held down the side button and then the green Power Reserve clock went on and that's it. I put it on the charger and then the Apple logo came on for a few minutes and when my watch screen came on again, it still said my battery at 90%. Like I said, it's only done it a few times in the last 6 months or so. Could I be hitting Power Reserve on accident? It's possible because the battery seems fine, lasts almost all day and doesn't really drain abnormally fast or anything.