I don't work in a grocery store, but one of my pet peeves is people who obviously have more items in their cart than the posted item limit for the express lane. Even more annoying than that are the people who claim that more than one of the same thing (say, 2 cartons of milk as opposed to 1) count as "one item" - no, it doesn't, if you've got 2 bags of salad, 2 cartons of milk, 2 bottles of salad dressing, 2 jars of mayonnaise, 4 packs of lunchmeat, 2 loaves of bread, and 2 boxes of cereal, and the posted item limit is 9 items. You have 16 items, not 9. I'd be fine with it if you're in the 15-items-or-less line and are over by one item, but when you've got almost double the posted item limit you're in the wrong line.
This is especially annoying when I'm on my lunch hour, I'm in a hurry to get back to work so I can eat my lunch, I've got exactly 2 or 3 items (say, a sandwich, a fruit and a drink) in hand, and you're standing in the express line with your month's worth of groceries. And then when I ask you politely if you mind if I go ahead, since I've only got 3 items and I have to get back to work inside of an hour, you snarl at me and tell me that "you were here first." Well...good point, but I'm not the one who's standing in the line where the posted item limit is clearly "15 items or less" (or whatever the limit is) and you're WAY over the limit, while I'm well under the limit and trying to beat to clock, to boot.
I try not to end up behind these people when I'm in a hurry, but sometimes it's unavoidable, and 9 times out of 10 the cashier will just take the person without pointing out that that person is inconveniencing others by bringing their ginormous cartload of 1 month's groceries into the express lane, thereby defeating the entire purpose behind the concept of "express."
Maybe it's me, but it doesn't seem like a smart move to alienate your main customer base when it happens to be the employees of the county offices across the street, who are usually in a time crunch and need to get in and out of the store pretty quickly.