Once I was lucky like that. I bought a 4 pair underwear pack from a store and scan it at the price check scanner. It comes as 2 pair multipack, which was on special, (and out of stock too) with the special price of two pack. So I bought almost all of them.
At that time (some 20+ years ago) each and every shop, not linked to their website, like modern days. So, whenever a special comes up on a catalogue, somebody in each store had to manually change the barcode prices. In my case, 4 pack barcode number must have changed instead of 2 pack.