That/which, I/me, and other similar grammar errors don’t bother me too much, since the usage and definition of those words are very similar. The errors are understandable, unless you’re a copyeditor or just a grammar geek. Plus, if everyone got those right all the time, I wouldn’t get as much proofreading work!

The ones that drive me crazy are you’re/your and their/there/they’re, since those words mean entirely different things.

I’ve had to teach myself to use poorer grammar in order for my first person narrator to sound like the person she is — what a challenge!

I think the thing that annoys me most, however, is one that Kristin mentioned, above: use of the nearly-correct word instead of the correct word. It all points back to whether or not the writer (and his editor!) has done his homework.

I’ve been told I’m the only one that does this, but I hate it when I’m really into a book, especially tense points, and I see a typo. I usually end up having to go back and reread the last few lines trying to figure out what was really meant (don’t judge me, I have dyslexia). It throws everything off and drives me up the wall.

Even with linguistic knowledge, a few of the common grammar mistakes listed still trip me up. I have to be pretty careful and double-check when I’m using lay/lie (or any of their past tense forms) or continual/continuous. I’m probably guilty of using “whether” in place of “if” at times, although that’s more me succumbing to common usage than true ignorance.

My biggest grammar pet peeves:
-“Irregardless”. What is irregard?

-Misuse of “jealous” in place of “envious”. They’re becoming interchangeable in common usage, but the true meanings of each word are nuanced and distinct. It’s become a joke with a friend to say we’re jealous of each other’s coats because, according to the proper definitions, that’s a pretty silly thing to be.

-“Affect” vs. “effect”. This is one of those ones that just seems obvious to me and therefore should be to everyone else.

-Ensure/insure mix-ups.