Sound over meaning

Further to JD's remarks on a tautologism that nearly got past us, earlier today I read straight through this: "...its importance to the economy cannot be underestimated". This phrase means the exact opposite of what the author intended.

And only minutes later one of our designers (highly creative chaps, of course, but not employed as wordsmiths) was checking a page proof for correct use of pictures and so on when he noticed the phrase "off my own back". He asked if it should read "off my own bat". He was quite right, of course; the error reminds us how easy it is to half hear a word or phrase and forget its roots.

For example, many youngsters say and even write the phrase "I should of done that" rather than "I should have done that". Having heard and used the elision "should've" they are guided by the sound without considering the meaning.