Profgrrrrl, over at Playing School, Irreverently, recently wrote on "taking it all with a grain assault," about students who hear a word or phrase but get it wrong when they try to reproduce it on the page: "grain of salt" becomes "grain assault," "dog eat dog" morphs into "doggy dog," and so on. Her readers added other examples in comments: putting women on "petal stools" (pedestals), someone looking for an "escape goat" (a scapegoat), for all "intensive purposes" (intents and purposes), and more.
I've seen "deep-seeded" for "deep-seated," I think more than once. That one almost makes sense, at least more than that escape goat.
If you like this sort of thing, check out The Eggcorn Database (for "acorn," of course), which includes: giving up the goat (ghost), chickens coming home to roast (roost), French (fringe) benefits, and 566 others. (And you can submit new ones.)
Sometimes the problem isn't mis-hearing a word, it's relying too much on spell check. I'm sure that was the problem when a student wrote a paper for me a year or so ago about the American colonists fighting against Tierney.
