Quantum Leap

There are (at least) three terms in common use in English that have come to mean pretty much the opposite of their original meaning.

Quantum Leap (n)

Colloquial meaning: A huge, metaphorical jump from one regime to another. "The consumption of organic foods took a quantum leap when Wal-Mart started selling it."

Actual meaning: The smallest possible step in energy levels that an electron, bound to a nucleus, can make.

Cherry Pick (v)

Colloquial meaning: To select only data that satisfy the conclusion you are trying to make. "Cheney and Bush cherry picked intelligence data on Iraq to justify the 2002 invasion."

Actual meaning: Use a cherry picker to raise yourself high into a cherry tree so you can pick all of the cherries you can see.

Show Stopper (n)

Colloquial meaning: A problem so bad (and usually unanticipated) that you just cannot do what you intended to do. "He wanted to use the computer, but the fact that the disk was corrupt was a real show stopper."

Actual meaning: A performance (usually by a singer in a Broadway Musical) that is so fantastic and wonderful the the audience stops the performance with their applause.