A: When it's a fixed phrase.
My mother has a bit of a tendency towards linguistic prescriptivism: in her mind, some things are wrong and some things are right. Like most of us, she can find sufficient justication.
One of her pet hates is the phrase "moment in time". To her, this is very wrong, because it's tautologous. What other type of moment can there be, after all?
Well, I just happened to read Prisoner of Zenda this year, as I thought it was on the reading list for the Cambridge exam First Certificate of English which several of my current students were intending to take. (Special thanks to About.com for having an out-of-date and undated list of books....)
The first chapter ends as follows:
"Colour in a man," said I, "is a matter of no more moment than that!"—and I gave her something of no value.
"God send the kitchen door be shut!" said she.
"Amen!" said I, and left her.
In fact, however, as I now know, colour is sometimes of considerable moment to a man.Clearly, "moment" here is nothing to do with time. I figure that this sense of "moment" must be the root of the word "momentous", meaning very important. Even though this sense of the word is now dead in common speech, "moment in time" has survived as a fixed phrase, so it is difficult to justify it as "wrong". (See also "moment of inertia" in physics.)
Another thing my mother objects to quite strongly is the word "bloody", and the history of this one is quite fascinating. Somebody somewhere along the line basically decided that the word was offensive (well, it has to be, doesn't it? Common people use it!) and then looked for why it was offensive. From there came the bizarre myth that it was swearing in the name of Mary, Jesus's mother in the High Christian traditions, and anyone who subscribes to a high church religion would consider that a very bad thing indeed, because Mary typifies virtue and purity. The trouble is, there is no attested process by which "by Our Lady" would mutuate into "bloody". And even more damning -- I'm told that other Germanic languages use (or used to, at the very least) cognates exactly like we use "bloody": both as a descriptive adjective (that shirt is very bloody) and as an intensifier (that bloody shirt is bloody awful).
Quite often, these days you'll hear UK English speakers decrying "Americanisms" creeping into the language on this side of the Atlantic, but very often when you look at the historical records and listen to old recordings you'll discover that these so-called Americanisms have been alive and well in the UK for centuries. Many of them are actually Scotticisms, borrowed into English in the US by Scots-speaking immigrant communities. Many others are simply dialectal variation within England. And a surprising number of them are in fact the most common form in use in the UK.
Denouncing another native speaker's language as "wrong" is very dangerous, because if you're the one who is wrong, you leave yourself looking like a prat. And no-one wants to look like a prat.