I am a nitpicky person by nature; I like things to be exactly correct. This is somewhat of a curse when reading stuff written by less persnickety authors who don't have professional editors or proofreaders. Having been involved in online communities for somewhere around 20 years, I have learned to ignore most minor errors. However, there are a few common writing faults that still really irritate me:
1) the overcorrectional "I", or as I like to yell at the screen, "Nominative vs. Objective, you fool!"
This fault is clearly the result of years of being told "It's 'Bob and I,' not 'Bob and me,'" and applying it too liberally.
"Bob and me went to the store" -> Wrong.
"Bob and I went to the store" -> Right.
But,
"Molly rode along with Bob, Frank, and I" -> WRONG! Wrong, wrong, wrong.
"You" are not the subject of this sentence (where the nominative case "I" goes), but the object of the "with" clause (where an objective case "me" goes). The easy way to know which form of pronoun to use is to test the sentence without the other person or people in it with you. Would you say "Molly rode along with I?" (I hope you wouldn't, because then I'd really have to smack you.) Clearly "me" is the correct pronoun in that case; therefore regardless of who else is included in the sentence, when you get to yourself, you should say "me" instead of "I".
2) The improper possessive apostrophe, or "It's not "It's", it's 'its!'"
For some reason, way too many people seem to think that using an apostrophe is the only correct way to denote a third-person possessive. In the case of pronouns, however, an apostrophe is NEVER used. It goes "MY or MINE, YOUR or YOURS, HIS, HER or HERS, ITS, OUR or OURS, and THEIR or THEIRS." No apostrophes at all. It is NOT "You're car", it's "Your car". It is NOT "they're loss", it's "their loss".
People seem to be especially confused in the case of "it's" vs. "its". I will just state for all to see:
"ITS" (with no apostrophe) ALWAYS MEANS "BELONGING TO IT." As in, "what are its properties?"
"IT'S" (with apostrophe) ALWAYS MEANS "IT IS." As in, "It's time to move on to the next item."
3) The improper dash, or "How not to hyphenate."
This one is a bit more esoteric than the first two. Well, okay, it's a lot more esoteric. But be that as it may, it's still not difficult to do correctly. Sometimes people like to include interjections in a sentence. Oftentimes--as with right now--one correct way to punctuate those interjections is with a dash. Specifically, it is with an "em-dash", a dash that is as wide as the letter "m." One should not use hyphen when an em-dash is called for.
The astute reader will probably be thinking, "But I don't have an em-dash on my keyboard!" That is true, and so in order to emulate the em-dash, the correct typographical representation is two hyphens together: "--".
The other thing to consider, however, is that you NEVER put a space on EITHER SIDE of an em-dash. And so if you write something like this:
Bob said - I'm just paraphrasing, by the way - that you should eat s**t and die.
Then you are not just wrong, not even just doubly wrong, but triply wrong, on each occurrance. (no space before, use two hyphens, no space after.)
The simple rule for this case is, If you're not creating a compound word (like "em-dash", "brain-dead" or "half-baked"), use an em-dash, and don't put spaces around it.
4)"They're" vs. "Their" vs. "There", or "Get it right, you buffoon!"
Really, it just isn't that hard. "They're" is short for "They Are." "Their" means "belonging to them." "There" indicates a previously-referenced location.
There are other grammar violations that are also annoying, but those are the top of the list. So if you want to really irk me (and show off your poor writing skills), just say something like
"You're car - it's tires are flat, so you have to ride with John, Frank and I, because their going too."