By the time layout work starts, text editing must be completed and the text must be entered correctly (see helpful information for clients). Layout work when adding visual material is a very precise work and the length of the text plays an important role  adding or deleting one or two sentences may require redoing the layout for the whole page, not to mention changing longer paragraphs or even chapters. Meanwhile, small occasional errors which are discovered during the final check are a normal part of the process and they will be corrected.