I'm not exactly sure how nitpicky you mean, but I'll try to answer.
I admit, I don't always catch mistakes. Grammar and spelling, as most people will agree, can make and break a story. All too often, if there are a certain number of mistakes in the first few paragraphs, I abandon the story. A few I can let slip by, because even a good eye can miss a few mistakes.
Going beyond that, though, plot points sometimes slip by me. Granted, if things don't make sense, I'll stop. Gross out of character reactions? I'll stop. I usually won't bother commenting on that particular story. But, then, I often forget to comment anyway, so that's not a big point.
The few times I've actually noticed a glaring error, I've let the author know, so they can fix it.
