This happened to a friend of mine while he was still just starting out in cadets. It was at his mess dinner, and he had just been promoted to Corporal, so he was very excited and a little out of it with pride. When he was tasked to go get something from the other room, he juumped on it.
Now, seeings as it was a special occasion, there were quite a few high ranking officers walking around. This guy, as he's passing through the room, walks by two Majors... one on either side. SO, up comes the salute... with both hands.