Malapropism is the term for the incorrect use of a similar-sounding word to the word actually needed. For example: She was getting more and more exaggerated, instead of the correct: She was getting more and more exasperated.
Another example: You need a new pair of testicles, instead of the correct: You need a new pair of spectacles.
Wikipedia has an article on malapropisms, including information about the origin of the term and many more examples.
Here is an interactive quiz on malapropisms and other misspellings found in The Guardian's comments sections.