I recently bought some cereal on sale - I purchased four boxes of the same cereal and all of the boxes were the same size.
When I got home and was looking at my receipt, I noticed that two of the boxes were 16 oz. and two were 14.5 oz. How sneaky! I paid the same price and the boxes were so identical looking that I had no thought to check and make sure they all weighed the same.
But, it only takes once to learn something like this - you can be sure I'll be watching more closely next time!