I cannot STAND it when people use the phrase "could of" instead of "could have" or "could've." I just don't get it, what do they think "I could OF done ___" means? Of does NOT WORK in this phrase. I actually have to stop talking to someone when I see them type that online because it bothers me so much.
And this isn't really a grammatical mistake, but one of my friends recently told me that he doesn't understand the difference between the meaning of qualitative and the meaning of quantitative. Even after I explained it to him several times. I had to put my head down in shame.