See, I think I'm quite good at noticing that stuff in other people's fic, it's just in my own that I get really sloppy sometimes. When I'm reading, mistakes like that tend not to bother me if I know that the author does know better and she's just made a mistake in her hurry to post or whatever, and I'll often send and e-mail or leave a comment about the typos. But if it's a consistent misuse it can really keep me from enjoying a fic. I especially have issues with commas and apostrophes. :)