Confusing “its” with “it’s” is my greatest grammatical pet peeve, and is a crime second only to bad kerning. If I had a revolver I’d reach for it. Not shoot, but reach.
“Its” where “It’s” should be … barely noticeable. I presume they’re using some input device where the addition of an apostrophe is a royal pain in the butt. In fact I myself wouldn’t go through very much inconvenience to insert an apostrophe where it should be
“It’s” where “Its” should be, on the other hand? That’s a paddlin’
How about where “its” and “it’s” are both used, but in precisely and consistently the wrong places? I’ve seen that, too! 
I agree that the improper use of “it’s” is probably the more annoying because my brain insists on expanding it to “it is” and it becomes jarringly nonsensical – “the dog happily fetched it is bone”.
I don’t mind it when it’s written down, but it drives me crazy when it’s not spoken correctly.
I’d think almost everyone knows “they’re” is a contraction of “they are” and would select the correct word if they thought about it. When a person types their instead of they’re or it’s instead of its, it’s often just an aberration caused by typing quickly. Conclusions drawn about the person’s intelligence from something like this are likely to be very wrong.
I know this from myself. It’s been 55 years since I won a spelling bee but I test myself with games and puzzles and am not yet demented. However I frequently type wrong words, often completely wrong words that are phonetically a little similar. Among many many examples, I once wrote “vowels” for “values” and “pretends” for “protects.” I see evidence that some other Dopers make similar errors.
I don’t think it affects my speech much; I’m not sure about handwriting since almost all my writing is via keyboard. I didn’t have the problem when I was younger; it started in my 60’s. I find the way to avoid this and other typos is to stare at the keyboard while I’m typing, monitoring where the fingers actually go!
I think it’s an issue of muscle memory. Once you’ve done enough typing, you do it subconsciously rather than consciously. And sometimes your subconscious leads you astray.
I’ve noticed, for example, that when I mean to type ratio, I often end up typing ration. I think it’s just that my fingers are used to following the letter sequence tio with an n.
I blame auto-correct.
Its vs. it’s is noticeable, but not infuriating. At least not compared to
[ul][li]Isreal.[/li][li]If you misuse the contraction of ‘you are’, your going to get slapped.[/li][li]The name of the external female genitalia is NOT ‘VAGINA’. IT IS ‘VULVA’.[/li][li]Other things that I will remember after the edit window.[/ul][/li]A tech writer friend of mine proof-read some documentation I wrote, and nit-picked some minor errors I made. I wrote back telling her not to be so anal retentive.
She wrote back, “Anal-retentive should be hyphenated.”
Regards,
Shodan
I also almost always write “ration” for “ratio.” (I did it just now, but corrected it.) Another word I always get wrong is “becuase.” (I was watching my fingers hoping to stop the typo this time, but my finger just had to go to that u-key. 
) I also often miswrite “Internet” as “Interent.”
Any typo can happen occasionally. I’d never infer something from a single mistake. But if you messed up it/it’s twice in one Twitter post, I might give you a pass. Mess up their/they’re twice in one Twitter post and I’m going to tell you about it.
I don’t say anything unless someone’s asked me to proofread an email or homework OR if it’s a published news story. I cannot stand to see people mix up it’s and its, their/there/they’re, then and than, etc. on a website for a newspaper or TV station. If your job is to write stories in English, by golly, you’d better have a damned good grasp of it. One station in Austin is just awful. No editing at all. I definitely say something in those instances.
I do feel that anyone who’s getting paid to write something should be held to a higher standard. What prompted this poll was an email from the History Book Club titled “Remembering Pearl Harbor on it’s 75th Anniversary”.
I would swear that growing up we were taught that its’ was also used in come cases, but I’ll be damned if I can find any reference to that now.
On the list!
My reaction isn’t on the list. I feel a little smug for noticing the error, then I keep reading.
Adolph Hitler’s speechwriters were the original grammar Nazis
Using apostrophe-‘s’ to pluralize a noun is more annoying to me.
The thing I hate about it is the randomness. I’m pretty sure you don’t think every noun needs apostrophe-‘s’ to pluralize it. So what do you think the rule is?
Sometimes I think people think the rule is “if it ends in a vowel, use an apostrophe, otherwise just add an ‘s’”. But then I’ll see other examples where somebody writes “free cat’s” or something.
Maybe some people think it’s just a style thing, like a stroke through a seven or a serial comma?
Anyway I do tend toward descriptivism in general but that one does bug me. It’s the inconsistency!
I voted “I don’t say anything. But I put that person on my list.” It depends on context, though - if it were one of my sons (hasn’t happened yet), or a student of mine (happens now and then), I will definitely red-pen it. Everyone else, though, I remain silent and they just drop a notch in my estimation of their education/intelligence.
I like to replace the “it’s” with “it is” and amuse myself with the resulting nonsense, e.g., “Turn the cup on it is side”
^ Adolf
Yeah, this.
Except, it isn’t amusing; it’s just something that I do automatically.