Angus & Robertson, in the glory days when Beatrice Davis was their senior editor, used to consider three typos per book was a reasonable amount, and that's still a target to aim at. Of course, there is huge potential for mistakes in any publication–all those letters, words, names, dates, facts, numbers and typography–and the closer you are to it the harder it is to see them. It's common for experienced editors to glance at a proofread page and immediately spot an error that the proofreader has missed. This is not a sneer at proofreaders; it just demonstrates how hard it is to detect errors in print. It's fatally easy to miss typos in the displayed type of titles and headings.

Some editors claim that the process of reading actually creates typos. The action of passing one's eyes over the text, they say, disarranges or moves the letters; consequently, the more often a page is read, the more errors there are. Despite considerable anecdotal evidence about this effect, rigorous scientific investigation has proved that it is illusory. Editors should, however, understand the operation of Muphry's Law of Proofreading, as stated by John Bangsund: