What annoys me etiquette-wise is when you end up in the closest bay to the scanner and everyone to the right (or left depending what side you're on) shoves their trays out before you can get yours. I've been left standing there while 2 or 3 people change over before I can actually get my tray out. Once after I got fed up I just shoved it forward and the guy next to me tried to tell me off. I told him I'd been there since 2 people before him trying to get the tray out.