If you are hiring me to proofread or copyedit your Word file, and that file has not been through a previous round of extensive and meticulous editorial revision, then I am likely to make thousands of amendments – correcting spelling, grammar, syntax and punctuation errors and inconsistencies, attending to layout problems with regard to spacing and paragraph indentation, tracking character names and traits, and implementing consistency across word forms.
I’d love to say, ‘I’ve made 7,686 revisions to your document, compiled 43 queries, spotted four problems with character-history consistency, noticed two character-surname changes, offered 118 suggestions for alternative wording, and I guarantee that, in spite of this extensive level of revision, I have not missed one single literal or contextual error.’
I’d love to tell you that all those mainstream publishers are wasting their money when they commission multiple rounds of editing and proofreading, and that the UK's industry-recognized professional editorial society doesn't know what it's talking about.
But I won’t, because I can’t provide you with that guarantee (I’m not a wizard!) and those publishers and the SfEP really do know their business!
In response to the question of whether perfection can be expected from the proofreader, the SfEP (the UK’s professional editorial association) says:
‘That is the aim, but perfection is rarely possible. By the Law of Diminishing Returns, perfection requires inordinate amounts of time and money. It is not realistic, but nor are some clients. Even when time is tight, they still want perfection while paying only for ‘good enough’. If they did not pay for copy-editing, the proofreader can only sort out the worst problems. […]
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.
Society for Editors and Proofreaders, 'Standards in proofreading'