Hi Anon No, using "of" rather than "have" would always be grammatically incorrect. However, it is a mistake that quite a few native speakers of English make when writing. When you say " would've been " (which is how native speakers of English typically pronounce "would have been" in conversation), the contraction would've is pronounced in a way that sounds very much like "would of" -- and that is most likely where the written mistake comes from.
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