It’s a question of literacy, John. In fact, I think it’s indicative of the so-called “brain drain” we have in this country. It’s a simple thing, and yet people still don’t get it.
Another one that drives me crazy is when someone doesn’t know the difference between:
your and you’re
there, their, and they’re
I learned it this way:
If you can’t substitute “It is” in the sentence, then it’s “its”.
“The cat’s got its tail caught in the door.”
“They say it’s going to be really hot this summer.”
Very simple.
I think the reason is this: People see words ending in “s”. Sometimes those words ending in “s” have an apostrophe. Instead of figuring out why, these people must think, “Oh, words ending in ‘s’ should have an apostrophe before the ‘s’.” And since they don’t want to seem ignorant, they’ll put the apostrophes in every word ending in “s” just to be safe.
I think it’s similar to people who think a quarter is equal to 0.25 cents. (Wish I could find the cents symbol in one of these fonts to really make the point, but oh well.) Often when things cost less than a dollar, a decimal point is added to the cost. People don’t seem to consider that there might be a difference if you use a dollar sign or a cents symbol, so they make sure to add a symbol that seems to be necessary. Occasionally, I’ve been tempted to try purchasing something whose price is given as 0.99 cents by giving the seller an entire penny and saying, “Keep the change!”
Does this happen in other countries? Do people write 0.50 p when they mean 50p?
I hesitate to make any judgment on the literacy of an individual based on their use of it/it’s, there/their/they’re, your/you’re, etc… Many people who clearly know the difference between all the various homophones occassionally make a slip of the keypad and substitute one for the other. Now, there’s a difference between this and consistently making the same error, but I know I see these errors here on the SDMB from people who obviously know the difference.
Hey, everybody makes mistakes, but when it is a consistent thing and the mistakes are liberally sprinkled throughout the post/ memo/ essay it is not an accident, it is a literacy problem.
Person 1: What’s that substance that the robot’s carrying?
Person 2: Its/It’s fuel.
Has Person 2 given any indication of whether the fuel belongs to the robot or not?
Touche!
Simon: OTOH, if you accept the use of “Robot’s” for “Robot is”, then the confusion people have is understandable:
Bob is going to school. = Bob’s going to school.
The book belongs to Bob. = The book is Bob’s.
Why do both get the apostrophe, but “it” does not?
Class has been relocated to MPSIMS.
Veb
It’s been relocated!! 
Or perhaps rather than a slip (hitting the wrong key), it’s just habit–automatically typing one homophone because it’s more common. Typing is a kind of robotic activity at times; you think of a word “accustically” and your fingures move without focused thought on what they are doing.
I think the reason that “it” came to be the only exception to the “-s is plural, and -'s is possessive” rule is that “it” has no plural. If there are two things you’d call “it”, and you want to refer to both of them, you say “they” or “them.” “Its” would be meaningless, so “its” came to mean “belonging to it” and “it’s” to mean “it is” or “it has”
My students do the extra apostrophe thing all the time. Finally I asked one of them why she did it. She honestly told me that she didn’t know how/when to use it, so she just stuck it in there, since she didn’t care enough to learn.
I saw a big sign for the housewares department at JcPenney that had the extra apostrophe. Given the style of the lettering, I’d guess it had been up there since the seventies.
The punctuation thing that is really starting to chap my hide is the notion that ''word" means word is emphasized. No. “” means quotes, underline or use italics, but quotes are for words that people are saying, like dialogue. They look at me as though I’m speaking Swahili, with their big, empty cow eyes.
I’m not sure why a comma/quote question is more appropriate here than in this thread, but here is where Moderator Giraffe is directing me, so here goes.
In the other thread, Eonwe said
According to the same reference & page as I quoted before (The Gregg Reference Manual), Par. 247b says
(Italics in original)
In case the above example is confusing on the screen, the word “rush” is followed by a period, single quote, double quote.
OK, fine. I knew that, but somehow, I’ve forgotten the variation about plurals of, well let me show you.
Do you know your ABC’s? or ABCs?
My dad is trixa…triski…tiskataska…he’s afraid of 13’s. or 13s.
ABCs
13s
I should add that there are times when an apostrophe might be necessary to avoid confusion. Take the baseball teams, the Baltimore Orioles and the Oakland Athletics.
These teams’ names are often abbreviated to O’s and A’s respectively. Putting in an apostrophe avoids the rather awkward appearance of Os and As.
Similarly, if i asked the question, “How many a’s are in the word abacus?”, i would probably put the apostophe in as i have done, because “How many as are in the word abacus?” might cause confusion.