While commenting on a sort-of-related post at The Common Room, Mama Squirrel heard for the first time about Egg Corns. Being a Squirrel, she naturally got very excited, thinking this was something to eat, but no. These Egg Corns are a recently-coined term for a particular language mistake that people make, like Spoonerisms and Mondegreens. The official Egg Corn website seems to be here, and you can see a whole catalogue of examples of this kind of error.
So what is an egg corn? It's a mis-hearing or mis-understanding of a word or phrase, often because the person is unaware of its origins. Sometimes a particular egg corn gets to be widespread. Mama Squirrel had already heard the example pre-Madonna instead of prima donna, which is on the Eggcorns website. But it might be just one person who misunderstands (like the egg corn thing, which came from someone thinking that's how you spell acorn). It shouldn't be a mishearing of a song lyric, though, because that's a Mondegreen. Other common ones are people saying that something "peaks their interest" or "touches a cord," or that you "pour over something." One might say that those are just misspellings, but the point of an egg corn is that there's also some misunderstanding of the phrase's meaning. Check out the website and you'll see what the difference is.
Mama Squirrel found a great example of an Egg Corn this past week in a book which shall remain nameless but which could probably have used some copyediting. The book mentioned the different kinds of dancing you could take--ballet, jazz, and flamingo.