Here’s something I’ve seen loads of times, even in professional publications:
“I should of known”
The ‘have’ has been replaced with an ‘of’.
I’m not going to bore you with terminology, I’ll just explain it instead, using a phrase we can all relate to.
You can say, “I have bought more beer”.
So, you can then say, “I should have bought more beer”.
But you can’t say, “I of bought more beer”. (Well, you might if you drank all that beer).
So you can’t say “I should of bought more beer”.
However, we often shorten (and write) this as ‘should’ve’ (or could’ve or would’ve), and this makes it sound very much like ‘should of’, so it’s understandable that this error is so common.
But it does then beg the question:
Can I say, “If I hadn’t’ve read this post, I mightn’t’ve known that!”?