I work at a pet supplies chain. My least favorite customers are the ones who bring over a basket filled with a ton of different flavors of Fancy Feast or some other shitty pet food, put the basket on my tiny counter instead of taking out the cans themselves.
Then they sigh over how long it takes me to remove and scan the cans. I don't even mind taking everything out of the basket myself - it's when they get irritated over how long it takes!
Even more annoying is when I get dumb questions about why I can't just scan one can x amount of times when they've bought different flavors, types, or even different brands. I don't think it takes a whole lot of common sense to figure that one out.
In general, I pretty much hate when customers assume they know what I can and can't do on the register better than I do.