Could have, should have, would have.
Could of, should of, would of.
I often see this error in writing, it usually happens when people are more familiar with hearing english than writing it down, when people say ”could have” it is usually shortened to “could’ve” now when you see that written you know that the ‘ve is a contraction of the word have.
However when someone speaks it, the words get slurred together, so it comes out sounding like ‘could of’‘ which is what people write down, that is gramatically incorrect and leads to some lovely lovely convoluted sentences like;
“She should’ve have not been so silly’”
Which is double contraction, but looks and sounds ok(ish) if you think that “should’ve” is a contraction of “should of”
The same rule applies to “Must have” and “Must of” . Though I do not usually come across the contraction of “must’ve” very often, it is usually either written “must have” or “must of”.
Must have is gramatically correct.