Grammar Police

Given the omnipresence of social media (blogs, Facebook and the like) in my life, I have begun to take note of some common grammatical errors. Now, I certainly don't proclaim to know all that there is to know about grammar, but there are certain things that get under my skin. For many of these errors, I think, 'Shouldn't you have learned this in middle school?!?'

But I digress. I thought I would give a little list of some common errors that I have seen, and also share an interesting site called Grammar Girl, a blog about grammar that I think I, and the rest of America, could learn a good deal from.

Here are my "favorite" (I can't really say that they are my favorites, because they make me cringe whenever I see or hear them, but you get my point):
1) Your/you're: Okay, people, I can guarantee that we ALL learned this in elementary school. The eternal question of whether to contract or not contract...It annoys me to no end when I see "I hope your having a good day" (which I do, with alarming frequency, on Facebook in particular). Okay, "your" is a possessive pronoun or adjective, depending on how it is being used. "You're," obviously the more appropriate choice in this situation, is a contraction of "you are". Done.
2) There/they're/their: Sort of along the same lines as the previous entry. "There" is an adverb (a.k.a. modifies a verb), "they're" is a contraction for "they are" and "their" is a possessive pronoun. To say, "Their going to the beach today" is incorrect.
3) Using apostrophes to make words plural makes me want to cry. When I read, "two medium pizza's" on a menu, I tense up a little bit. Adding an apostrophe makes a word POSSESSIVE, NOT PLURAL(also, if a word ends in a -z, which my last name happens to, or an -s you add -es to make it plural).
4) The dreaded double conditional: This probably bothers me mostly because of how hard the tenses were drilled into my mind in my Spanish and French classes. Certain tenses may be paired together, and others may not. Something that I have recently noticed people saying is, "If I would have known______, I would have done ______". This is called using a double conditional. If the speaker used the past perfect tense in the first verb clause, the sentence would be correct, and would read something like, "If I HAD known ______, I would have done ______".
5) A funny/sad? anecdote: I received an email from a teaching agency that read, "We want to insure that you have all of the information you need...," or something to that effect. Ensure, people, ENSURE. INSURE is for insurance. And this was from a teaching agency...sheesh!

How about all of you? Are there grammatical errors that make your blood boil? Or am I just neurotic? (I acknowledge that the latter is quite probable.)