According to this story in the Denver Post, a 19-year-old college freshman used barcode printing software to print barcodes for cheap products which he then swapped with the barcodes on expensive products. He was caught because he returned to the same store twice. Using legitimate software available on the internet, he entered the barcode number for the cheaper products, created the new barcodes, and swapped them in the store. He bought a $250 iPod accessory for $25 (based on a CD player barcode) and a $149 iPod for th eprice of $4.99 headphones. The latter event caught the notice of Target security.
(In theory, RFID tags will make this type of fraud much harder, both because the tags are harder to duplicate, and because every item, not just every product, will have a unique identifier.)