My favorite mis-used expression of this type is the term "quantum leap." A "quantum" is the smallest amount of energy you can have, and a quantum leap is the smallest possible change in an electron's energy. So a quantum leap means an incredibly small change (although a discontinuous change, which is where the word "leap" comes in). But somehow in ordinary conversation it started to mean a huge change - exactly the opposite of the original meaning. So if someone says we will make a quantum leap next year, they'll likely be right according to at least one of the definitions!