I critique works-in-progress for other writers and participate on writing-related forums. Typos aside, I see a few common errors made by people using the English language as a tool (to tell a story). In my mind, language for a writer is like the toolbox for the carpenter. He can't build a house without knowing how to use a hammer and when it's better to use a screwdriver. Writers can't tell a story without understanding how to put a sentence together.
Writers, there's no excuse for thinking nouns are made plural by adding 's. More than one author is "authors" and not "author's." This is a rule we learned in first grade. We use this language every waking moment of our lives. Self-published authors in particular need to be diligent about proper grammar and spelling. Their reputations may be on the line.
If you find yourself being corrected by readers on grammar problems, it may be time to brush up. Check out the Guide to Grammar and Writing -- they have some fun quizzes that will test your knowledge.