There are some errors that drive me nuts. I see so many people write, "I thought to myself." Er, unless your character is telepathic, who else would they be thinking to?
Wrongly placed apostrophes grow like mushrooms. If you see the wrong it's in a story, it won't be long before you'll also see your's and her's.
Miss Snark and others had a field day a few months ago on her Crapometer - the problem was the people who talked about having written a "fiction novel". A novel is fiction. That's like saying an automobile car.
I thought it was obvious. Then today I received my email newsletter from Borders and it has a discount coupon on it - 20% off a fiction novel.