could of, should of, would of/could have, should have, would have: Common Errors in English Usage Entry for Tuesday, May 29, 2012.

could of, should of, would of/could have, should have, would have
This is one of those errors typically made by a person more familiar with the spoken than the written form of English. A sentence like “I would have gone if anyone had given me free tickets” is normally spoken in a slurred way so that the two words “would have” are not distinctly separated, but blended together into what is properly rendered “would’ve.” Seeing that V tips you off right away that “would’ve” is a contraction of “would have.” But many people hear “would of” and that’s how they write it. Wrong.

Note that “must of” is similarly an error for “must have.”