Most (if not all) of us have likely heard the phrase "dead as a door-nail."
However, I have noticed that a large portion (ok, all) of my American university students of the last 5 years erroneously think that this phrase is instead "dead as a doorknob."
- In fact, most of my students are unsure what a door-nail is (though many can guess).
I assume that if my students represent a broader trend, there is a chance that the idiom might actually permanently "change" from using doornail to doorknob in the not so distant future.
My question: Is there a word or phrase to describe an instance in which an idiom actually changes due to a change in common (mis)usage?