I find this (very) frustrating as well. It just boggles my mind that UA doesn't do anything to police this. Not only have I had the experiences that previous posters have had, I've actually had the experience on an EWR - DCA flight where the f/a brought a cup of ice and a soda to another f/a who was sitting behind me. I find that the "too short" rule applies either in the morning or late evening, when someone is "too tired."