Authors will often ask if I can promise them an error-free manuscript at the end of the editing/proofreading process. It's a fair question. Editing and proofreading, when done properly, are a significant investment in time and money. I can understand why there might be an expectation of guaranteed perfection. Unfortunately, with the best will in the world, it is an unrealistic expectation.
This is not because professional editors and proofreaders are bad at their jobs; it is because they are only human. A computer program capable of making all of the intricate judgements that editors and proofreaders have to make every second that we are working for you, has yet to be created.
The work we do is subject to perception and involves many layers and levels of checks. We don't just read the words; we also examine the style, the punctuation, the grammar, the spaces in between, the consistency, the context, the flow, the legalities. We love the intricacies, the multi-facets; we revel in the challenge; we aim for perfection – but we cannot guarantee that no error will slip through or that everyone will agree with the decisions taken.
Perception of perfection is one factor. What one person perceives as correct, another might question.
There are various styles that we can work to which have different rules. Sometimes I work to industry standard style guides (e.g. New Hart's Rules, The Chicago Manual of Style) and other times I work to house style or the author's own style preference. Anyone who is used to working on or reading books published using industry standard style guides will perceive the house style deviations as incorrect but I will have followed my brief. The client always has the final say.
The rules of grammar change from generation to generation.
Some readers have a preference for an informal style of writing, whilst others like the formal approach to language, grammar and punctuation.
A person may have grown up in a different part of the English-speaking world to where your book is being published and be used to alternative spelling.
There are many factors that can lead one person to view something as an error whilst someone else will see it as acceptable. I will have followed your style and brief, worked within the context of your voice and genre, but not everybody reading the edited/proofread text will agree with the choices that have been made.
The volume of checks that need to be carried out is the other factor. Traditional publishers have always employed various editorial professionals at different levels of the editorial process. They don't take on this expense for the fun of it; they do it because it is necessary. Experience has shown that the more pairs of objective eyes that look at a manuscript, the better chance there is of ensuring every layer has been fully scanned and corrected. Even then, some errors can still be missed.
An editorial professional will have to juggle typos, punctuation, grammar, spelling, format, style, legal issues, consistency, fact-checking and a myriad of other tasks. If you only have the budget for one professional to do all of this, it is highly plausible that something could slip through the net.
Here are some figures courtesy of the SfEP (their article 'Will they make my text perfect' can be found here)
A good copy-editor can be expected to pick up 80% of errors, a good proofreader can be expected to pick up 80% of what's left.
An experienced professional proofreader, reading a copy-edited typescript, should be able to spot and deal appropriately with at least 80% of all errors but at least 90% of typos – other things being equal.
By this reckoning, there is a chance, not a certainty, that as many as 3% of the total errors could survive the editing and proofreading process. So, why bother?
People will judge you on the quality of what you put in front of them. (SfEP)
People will not take you or your message seriously if it is unclear, inconsistent or poorly presented. (SfEP)
You are asking people to spend time reading it, and it is simple courtesy to smooth the reader's path. (SfEP)
The quality of your manuscript will still be significantly better than it was before a professional editor/proofreader worked on it. (Athena Copy)
Always know that professional editors and proofreaders will be aiming for perfection, we want it as much as you do, but it would be unethical for us to guarantee it.
There are some things that you can do to help in your quest for perfection:
Use beta readers to help iron out the 'big picture' issues before paying for a professional editor's time.
Read through your manuscript yourself before sending it for editing or proofreading. Nobody knows the story better than you and you could save yourself a lot of unnecessary and costly queries.
Hire a line editor/copy-editor.
Make your revisions and read through your manuscript again.
Hire a proofreader, preferably someone different from your line editor/copy-editor (if budget allows).
If you cannot afford to pay for a proofreader, ask a trusted friend to read through your final draft to make sure that no new errors have crept in during the post-edit revision process.