I've heard this referred to as "Herblock's Law."

What's your favorite example? Here are a few of mine:

Marmot Precip Anorak...nice piece of lightweight gear, several ounces lighter than the jacket. In fact, they could have made it a jacket with a zipper without much weight sacrifice.

Pur used to make a filter that screwed onto the top of a Nalgene bottle and used small, nearly weightless filter discs. More expensive per quart filtered, but the discs were so light you could easily carry a spare or two. And you didn't have to lay out $50 for a new filter cartridge. The actual pump part was similar to the "Hiker", so flow rate was good, and If they wanted to, they could have made it without the screw-on adapter, making it even lighter. I think it was about 8oz.

Penny's "Plain Pocket" Jeans...50/50 cotton/polyester, ideal for those of us who like jeans for hiking. Now, instead, I pay $85 for hiking pants. Grrr...