To that one guest: when I call for back up on the front lanes, I don't need your opinion on it. Just hush up and let me do my job.
I like to mess with those types of people.
"We need one additional to the front registers please"
"YOU NEED MORE CASHIERS!"
"Well thank you for stating the obvious Captain Obvious. You have now selected yourself to be the one person who spends 5 minutes waiting to check out! Enjoy!!"