Wrong Ball

I hit the wrong ball today for the first time in my young golfing life. We were playing as a six-some and it was cold and rainy. I'm easily the shortest hitter in the group so after we all teed off on the long par-5 7th hole at my club I started walking towards the nearest ball in the fairway. Seeing it wasn't mine I naturally proceeded to the next ball and, trying to keep things moving, quickly went ahead and hit it. Turns out it wasn't my ball that I hit--I had actually out-driven two people in my group and was the 3rd shortest ball in the fairway. Two stroke penalty, I would go on to make a 7 on the hole. Didn't cost my partner any money as he thankfully went ahead and made par (net birdie) to halve the hole for us. But I didn't give us a chance to win the hole either and it was embarrassing to boot. From that point forward I have amended my pre-shot routine to include the question "is this your ball?"