When you work retail you encounter a large cross section of the general public, from the people who give you hope for humanity to the absolute dregs, and you really get to see how diverse people can be.
But bad customers tend to be pretty similar, easily broken down into, say, twelve different archetypes. Hey, whaddya know!
Jacob Andrews of For Lack Of A Better Comic fame has created an illustrated guide to the 12 Terrible Customers At Every Retail Job for CollegeHumor, how convenient!
Now those lucky folks who've never had a retail job can see why the retail workers they know are always complaining about customers. Because we're all "The one normal person you'll see all day" type, right? *wink*