Let me list some of the things my expert proof-reader found in Dear Agent (coming on Aug 10th!):
- Places where I'd got a very slightly different wording in the contents from the actual wording in the heading, because I'd changed my mind about the heading but not the contents list.
- I'd used the word blind-folded with a hyphen twice and without twice - the examples were far apart and to notice I'd have to remember to write down every single word that could have two forms as I came to them, so that I could check each one. There are LOADS of words that can be hyphenated or not, equally correctly, but you should do it the same each time.
- I use British spellings (eg realised not realized) but sometimes my computer auto-Americanises - my proof-reader had to notice.
- There were some things the meaning of which she wanted to check, because she wasn't sure it would be clear to everyone - this is a crucial task for a proof-reader and requires intelligence.
- She noticed a couple of bolds or italics which she questioned.
- She noticed that I seemed to have contradicted myself at one point. I hadn't, but I agreed it wasn't clear, so I adjusted.
- I'd capitalised something in one place but not another.
- I might have used curly quotes in one place and straight ones in another.
- Spelling and punctuation errors, of course.
- Wrong layout or line-spacing.*
- Inconsistent font/size in headings.
- Places where headings appear at the bottom of a page, or other uglinesses of layout.
For example, too much white space is a nightmare in some ebook formats. And, as Dan points out in the comments below, widows and orphans are irrelevant because each reader/device can be set up differently. When we read ebooks, we have to accept that sometimes a heading, for example, will appear at the bottom of the "page", ie screen.
I told my proof-reader to ignore widows and orphans and line spacing. I then put in my instructions for the formatter, and I always give her discretion about line spacing, within reason. The formatter is not only a highly expert ebook formatter, using full html, but she was also a trained type-setter. So, I have all bases covered! And there will doubtless still be something that slips through!)]
It's a skilled task. It requires patience, knowledge of correct and varied uses, and a very meticulous way of working.