I had the same thing happen with a pack of photo corners I bought from a discount store. They too were not taken apart properly. What is more is the clerk gave me what I knew was too cheap a price on them because she couldn't find the UPC to ring them up properly. This was after I had already indicated that the price in the location had been such and such and not the price she was ringing them up at. So, when I got home and discovered that I had multiple packs of them bound together I felt a little guilty. But I figured that at the time I had tried to be as honest as I could be and calling back about them would probably be more trouble than it was worth.
Of course I am also the type who will let myself be double charged for things or put up with faulty products without complaint... many times anyway. As has been said here already, I figure it averages out. And it isn't the same thing as intentionally walking away with something you knew you were paying too little for. Of course if you absolutely can't live with the guilt, there is always the phone... you can call them and tell them what happened and try to set it right.