In language, there's no such thing as a common error
This is a statement mired in controversy. It wasn't me that first said it, but I agree with it... with one caveat: we're talking about native language.
For many, many years, grammarians and school teachers would hound us for saying things wrong. As a child, I was constantly "corrected" by my mother for asking for permission with Can I...? instead of Please may I...? or for saying if I was you... in place of if I were you....
So when I studied English at university it was very heartening to find that modern linguistics considers everything that is said by a sizable chunk of the population as acceptable language. And of course this includes both Can I...? and if I was you....
What triggered this post was seeing an article on the Register about a grammatical error in a BBC headline: Phone-hacking: the other news you might of missed.
This is one of those "errors" that's now common enough and consistent enough that we may have to stop calling it an error.
When I suggest this, people often recoil in horror. "But it's the perfect tense," they cry, "logically it must be have." (And yes - I know that perfect is an aspect, not a tense, but pointing that out at this point would seem like cheap point-scoring so I generally let it lie.) But since when was language logical? You must and You have to are logically equivalent in some usage, but when you negate them you get two very different things: you mustn't and you don't have to.
The thing is, logic aside, we have empirical evidence that shows people's brains don't see it as have -- the errors themselves stand as proof of an emerging norm. Rather than fretting about the logic of have=perfect, we should be paying close attention to the thought processes behind this change and trying to make the way we right English match the way we speak it.
This does not mean that we have to accept might of, could of etc. No, because there is an existing mechanism that rids us of this problem: contractions.
Contractions are mostly hated by our schoolroom English teachers, but they are gaining growing acceptance. We're allowed can't now, where my primary school teacher insisted on cannot, and even I'm where my teacher insisted on I am. Yet we're still told off by teachers and editors if we try to use could've or coulda, should've or shoulda, might've or mighta. But these are what we say. Our habits of speaking have gradually reduced the auxiliary have to something more of a fusional element, a suffix, than a word. It is only when a writer is expected to write "in full words" that might've becomes might of, so why not simply accept might've? It would eliminate both the error and the controversy, and would say several pedants a few more grey hairs....