I don't remember the exact wording of the question, because it was at least a year ago that I heard it. But it came up once when I was on a writers' panel, and obviously it has stuck with me.
The question was along the lines of, "If you've made a mistake with an editor, can you ever approach her again, or are you blacklisted?"
I can't speak for editors or agents or even for all writers. I can't really speak for anyone but myself. However, this was my answer, and I suspect (and hope) that it applies widely: It depends on what you mean by "mistake," but I think blacklisting is rare.
There are small mistakes, and then there are really huge errors in judgment. Most of our mistakes fall into the first category. Did you misspell her name? Send the wrong version of a manuscript? Did you say, with rookie hubris, that your manuscript was like [fill in latest bestseller title] only better? Write your query in hot pink letters because you thought it would be eye-catching? Ask her, at your very first writers' conference, if she would take your 1000-page manuscript home with her? Don't get me wrong: these are behaviors to avoid if you can. But if you've done anything like this, either through a brain freeze or novice enthusiasm, I frankly doubt anyone will remember. It may make you cringe now, but the editor has seen so many queries since yours that the ones she passed on have probably melted together in her mind. Nobody's perfect, and I think most people have the compassion to see innocent mistakes as just that. The important thing is not whether you've ever stumbled, but whether you've learned. The point is: what does your manuscript look like now? How does your latest query sound? Are you cultivating a professional attitude now?
Of course, there are behaviors that could put off an editor permanently--stalkerish behavior, for example, like calling an editor at home or bombarding her with email or phone calls, or responding with extreme anger to a rejection. But I think those are much, much rarer than the innocent flubs we all make at one time or another. I would hope we can extend some understanding to one another, and not sweat the small stuff.