Having worked retail (though not Wal-Mart), I can vouch for KellyAdmirer's take on "customers..." I've had it happen to me, and I've seen it happen to others countless times.
Fortunately, about 80 to 90% of the customers I've dealt with were pretty decent people....it's that 10 to 20% that gets to you.
I had one woman actually get in my face once about a transaction her son made, while she was no where near, not even in the store. She accused us of cheeting him...though we went overboard trying to help the young lad spend his money wisely...which he decided not to do.
So instead of taking Jr. to task, she yells at us for taking advantage of her young, precious innocent boy...
Unfortunately for her, she made me the target of her little tantrum....and for the first time in MY LIFE, I gave a "customer" a dressing down the like of which you only see in Marine boot camp...
The only time I ever lost it, but this woman was a real piece of work...
Needless to say, she never came back, but 3 of the 7 people in line became repeat customers - because I was sooooo nice to a rude customer.