Proof readings is one of those things that writers have to do but sometimes fall into the biggest elephant trap of them all – familiarity. The writer is familiar with the book, they wrote it, so they are familiar with it and “know” what the text is supposed to say and read that instead of what is actually written.
I know this from personal experience on two fronts.
Firstly, there are the every day typos or words that are the wrong word because the computer makes assumptions and corrections and if your key strokes are slightly wrong, it puts in the wrong word. These should be easy to correct if you can read what is actually there and not what you think is there. This is a problem one for me because I am a “speed reader”.
The second problem can be far more difficult, and these are continuity errors. Subtle ones, not big howlers like the man character head red hair in the beginning and blond hair at the end, no, I mean actual problems with the text. I shall give an example of one of my own that I only “realised” after about the 6th draft and 2 reads from a beta reader.
In the beginning of the book, my two villains had a knife each that they used in an attack. They got left at the scene for another reason. But, a good way further into the book I have the police finding their knives in their stash beneath the bathroom floor. How can this be they do not have their knives, but because there wasn’t a “mistake” in that passage of text, I didn’t see the error.
This is where you have to be absolutely sure that you are reading the story correctly and understand the plot all the way through the book. It would not have looked good in a published version because the reader would have spotted it straight away and would then have been rightly critical of the work.
So, be sure you know what you are reading and truly understand your people, plot and perhaps you will not get caught by a big howler.