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.”