Typos are difficult to find for some reason. Years ago when our company produced our very first printed brochure we had every employee read over the source document and sign off on any corrections that needed to be made. Once everybody had given the "OK" on the entire document, we sent the thing off to the printer.

There was high excitement on the day the boxes arrived by UPS with our new brochures. We held a party to celebrate. We opened one of the big boxes and THEN everybody noticed that the company name was spelled wrong! Nobody had checked the obvious stuff like company name.

Also, our minds have a tendency to read things that we have produced as correct even when it is not.