Shrinking pack sizes and supermarket pricing oddities

Take care looking at price labels.
I picked up some Physical Milk at our local Woolworths Supermarket which was priced as special on the shelf label at $3.91, 20% off its normal price. After going through the check out, the charge was at the full price of $4.89. On querying one of the store managers checked the shelf label, removed it. In very fine print at the bottom the special was only till 24th January. This day was the 30th January, six days later. I was given a refund, but how many would be caught?