I spent a few hours today proofreading a book by a client published 12 years ago (first edition) and not long ago (second edition). She is going to turn it into a digital book and wanted the errors she's found corrected. When I did the original edits (and I do mean plural) all those years ago, we were working on paper. I'd print out her document, mark up the pages, and send it to her to input the changes. Along the way, she'd miss some or make other changes or complete rewrites of sentences and paragraphs that never came my way again.
So in a sense, I was her editor and for several versions, but there was no final proofread. She touts my skills regularly, refers people to me, but at the same time, I know her book isn't my best work. Because her final version isn't my work, or is only partially my work. And that's difficult to explain.
Another curious thing: I haven't read this book for a number of years but I have chiseled into my memory somewhere many of these sentences. They're surprisingly familiar and because they are proofreading is more difficult. I find myself skipping ahead and having to go back and read word for word.