they can't clean up after themselves.
...allow me to expand on this. one thing i've found is that for some reason, people love to trash the stores they shop at. i'm sure you've all seen the people who will just dump an armful of goods wherever they want, or the person who lets her kids pull stuff off the shelves and leave it sitting on the floor.
as someone who's spent a lot of time cleaning up after full grown men and women, i've got to say that it's one of the most inconsiderate things someone can do. if you've ever worked in retail, or any other similar job, you know exactly what i'm talking about. one or two things here and there are no big deal, but there are some people who feel the need to wipe shelves clean and leave everything scattered all across the floor - and these aren't kids who are getting a good laugh out of it; they're soccer moms who want the one item in the back, and are too lazy to get it without making a huge mess. i'd hate to see what their homes look like...
have you ever heard someone say "it's okay, they have people here who are paid to clean that up"? well, they don't. the task usually falls on someone who is far too busy to clean up after you. fortunately, i've moved on to slightly better things, and i don't have to deal with it nearly as often, but it just irks me to see how some adults behave themselves.
anyways, i'm sure that there are plenty of you that can relate, and i've love to hear some 'customer from hell' stories