I had a colleague once who bought a pack of cookies that said '10% free' or something like that. I turned out though that only 10% of the initial volume had been added (hope that explanation makes sense - it's a variation on the issue Steve44 is describing). Anyway so he wrote a letter to the company pointing that out, they send him back an apology letter and a box of free biscuits.

Moral of the story: pay attention in math class kids. If you don't, one day you might have to hand out free cookies.