Edited to Perfection?
Authors often assume that if they pay for an editor then their book should be 100% perfect. In reality, that's pretty much an unattainable dream. I have never read a book that didn't have at least one thing I would change; whether it was as miniscule as a double space, or as major as a character's name suddenly changing mid-way through the book.
When you think that editors are responsible for not only every word, but for every typeset character (and every fictional character's plausibility, dialogue and consistency in persona), every symbol and all of the spaces between them, as well as the correct arrangement of words, characters and spaces -- that's a lot to deal with right there. However, editors are also assessing line length and removing "tombstone" spaces; determining paragraph length; ensuring chapter length is consistent and that chapter ends create a "cliffhanger"; ensuring chapters link successfully to past and future events in the text; and removing non seqiturs, widows and orphans (phew!). In effect, editors are responsible for avoiding or fixing many thousands of potential errors (millions in some cases). They do all of this using a set of rather loose principles and guidelines: grammar, spelling, diction and punctuation, few of which are always "black and white". The flexibility of grammatical "rights and wrongs" also differs between genres, with literary fiction usually being the most forgiving. The rules editors live or die by also vary depending on house style and language of choice.
So, basically, you'll rarely see a 100% error-free book, or, rather, what is 100% error-free to one editor might not be so to another. In fact, I'd challenge you to find me a book of more than 60,000 words that is 100% perfect by any editor's standards. Even if it is, the next year something might well happen (either a change in accepted grammatical style, new scientific research or a momentous historic event) that leads to your book needing a change at reprint. That's not to say that editors aren't valuable, but that, as in most things in life, perfection is a pipe dream. The best we can hope for is dedicated professionalism and a commitment to being as close to "perfect" as possible.