I think it's a fundamental law in tech writing that glaring typos and errors are never noticed until after the manuals have been sent to the print shop. Entire teams of skilled people pore over the drafts to ensure the highest quality. Yet it's entirely probable that, when the proofs are returned by the print shop, the first thing you notice is that the name of the product is misspelled on the title page.

Sometimes you get a good one. In one guide, the table of contents showed the heading "Duplicates..... 176", which was immediately followed by "Duplicates..... 176". Oh, the irony.

Usually the mistakes are things that are so obvious that none of those several checkers bothered to even check. Like the page numbering that mysteriously restarted in the middle of the book. Or the page header that showed the title of another manual. Or the fact that hundreds of instances of the word "function" were spelled "fuction". Blame that one on search-and-replace.

There's really nothing you can do to avoid these things. Besides, of course, beating your head against the wall and shouting Doh!! when the same glaring mistake is printed ten thousand times.