The problem with proofreading is that very often you read what you expect to be there rather than what is really there. In my book the Devil's Own there was a line which was supposed to read "He turned to face (the island called) Batavia's Graveyard and raised one clenched fist in a grim salute." The manuscript went through editing, copy editing and typesetting. After the typesetting my editor proofread it, I proofread it and I got a friend to proofread it. Despite all that, what eventually appeared in the book was "He turned to face Batavia's Graveyard and raised one clenched fish in a grim salute."
I like to show this to kids when I do workshops or author talks to emphasise how important careful proofreading is, and you would be surprised how many of them have read the book themselves or even had it read aloud to them in class and no-one - including the teacher - has ever noticed it.
That's hilarious! 
Obviously it wouldn't work on a whole manuscript, but I used the reading it backwards method when what I wrote was important. I found it allowed me to focus on each word rather than the whole sentence.
