Eggcorn

noun.
an idiosyncratic substitution of a word or phrase for a word or words that sound similar or identical in the speaker's dialect (wikipedia)

I came across the term eggcorn from this story on All Things Considered and I think it is a great example of how interesting it is to watch the evolution of language!

Some eggcorns you are probably familiar with:

•for all intensive purposes instead of for all intents and purposes
•old timers' disease instead of Alzheimer's disease
•boggled instead of bogged
and so much more at The Eggcorn Database!