Two often-made mistakes:
"For all intensive purposes."
Well, that just doesn't make sense. But when you say it the right way, it's easy to hear how this mistake can be made. It should, of course, be "for all intents and purposes." [link]
"She really thinks that? Oh, she can be so diluted."
Yes, well, diluted means to make less concentrated, thinner, lessen the force of or decrease in value. Almost sounds right but not quite. The word you want is "deluded." When you are deluded you are deceived, tricked, or mislead. And, properly, what you probably mean is self-deluded. "Deluded" is a close cousin of "delusional." [link]
Are there any mistakes you know of that you hear a lot?