Mostly I take issue with the disappearance of "have."
Now, I know that "I could've" and "I could of" are pronounced in roughly the same way. This, however, does not prevent "I could of" from being totally meaningless.
I remember having to memorize a list of all the common helping verbs for school. "Of" is not one of them. It does not change the sense or tense of any verb. It is a preposition. It has no place in the predicate.
This error wouldn't bother me quite as much as it does if it were confined to illiterate eighth-graders. But I saw it on National Review Online the other day (and not on one of the blogs, either). This is inexcusable.
Have we forgotten how to teach our children their own language?