I'd be highly astonished if there were any truth to that rumour. The OED is the guardian of the language. By its nature, it has to be conservative, a little bit behind the language as it's used. And of course, what is said, daily, in speech, cannot always be accepted as the norm. I've had (English) students insisting that it's ok to write “I was sat in the corner of the pub” in an essay. Yeah, people say it every day (I even do, from time to time, when my inner policeman is off duty…). I just wasn't having it from them. “I was sitting in the corner”, or “I was seated in the corner”. I mark it as wrong, and will continue to do so to my dying breath, and if it annoys the youth — tough. They all shouted and screamed, “Oh no, Mr Lovebites, you need a long stay in the UK to find out how people really speak now!” No I fucking don't. I know how people in the US and the UK speak, thank you very much… And I don't care if anybody calls me a old fogey. Bring it on. Anyway, it's well established that I am an honorary member of the FOC brigade on here.
Speaking and writing (in an academic context, that is) — different gravy, as Sergio famously said.
“Must” followed by “of” simply makes no sense.
“I must have been mistaken”. Fine.
“The Queen of Sheba”. Fine.
“The nub of the matter”. No problem.
but “I must of done it.” ?? What the actual fuck?!
Edit: if this post seems to have an unduly combative tone to it, it occurs to me that it might be because I just picked up thirty student papers to mark yesterday, and I'm revving up for them… ;-)