Most of the time my editor will spot a "minor correction" in my story. ("Didn't you spell her last name with two Ls?") Sometimes she misses the minor stuff. But when I go back later to re-read it, I'll catch the gaffs (Sheesh, she has blue eyes! Why did I put green here?) And worry that my readers will catch those gaffs, as well.
So I keep a journal now. (Yes, that's a real picture of it, complete with my notes on the story I'm currently writing.) In it I keep pertinent information such as my hero and heroine's full names, eye color, hair color, build, special names, words, and phrases. I also mark dates (Did they first meet three years ago or five?), chapter headings (so I don't repeat myself), and whatever else I feel I may need to refer back to. (Once the book is finished, I mark the first page with the book's title on a post-it note, in case I do a sequel and need to review.)
In short, I'm going to do my darn-dest not to make any of those small, non-grammatical mistakes.
(And, yes, those are real coffee stains. LOL!)