A light on over your register means "yes! I'm open!" to practically everyone who shops in a department store. So when I walk up to your register and see you standing at the food counter jabbing your mouth, and politely wait a minute for you to come and help me dont look look at me like I'm a pain in the ass trying to ruin your day when your friend informs you I am in your line. And when you snip "I'm not open!", dont roll your eyes when I counter "your light is on". Walking away and leaving me standing there with my jaw dropped and your manager rushing in to take care of me is probably a good way to get fired.
(And yes, I was fully prepared to go to the other register that was open, so I didnt stand there like an idiot afterwards waiting for someone to give me service-just thought I would add this in case someone might think I was being a crazy customer).