Nobody expects the Grammar Inquisition!):
Your is possessive, while you're is a contraction of "you are."
Both words used in a sentence: You're pissing me off with your ignorance of proper grammar.
Lose means to fail to keep, while loose is the opposite of tight or constrained.
Both words used in a sentence: If you continue to maim the language, I will lose my mind and let loose a stream of profanity.
Their is possessive, there is a location and they're is a contraction of "they are."
All words used in a sentence: Their improper use of grammar guarantees that they're going to rot down there in Hell.
Saw is the past tense of "see", while seen is the past tense of "see" that must be used with an auxiliary verb like "have."
Both words used in a sentence: I have seen a lot of grammatical errors, but when I saw what you did, I wanted to punch you.
Its is possessive, while it's is a contraction of "it is" and its' is not a word.
Both (real) words used in a sentence: Due to the death of the English language, it's official, the world has surely come to its end.
Okay, enough. There are other infractions (for instance, it's supposed to not suppose to) that make me want to go on a killing spree, but I'll stop here before I have an aneurysm.
So, tell me folks, what common grammar errors make you cringe?
*Of course I am always willing to give people the benefit of the doubt if English is their second language. I certainly could not converse fluently in any language other than English, so I have great respect for individuals that are bi- or multilingual. Also, I am not talking about people who have the occasional mess up. We all have unintentional lapses in literacy on occasion. I do not claim to be infallible, but I do try my best to appear remotely intelligent. Now that I've said that, I'm sure someone will find some blaring mistake in my rant above.