Last week I found two 2's in the same sudoku square and another square where it would be impossible to put a two. In another issue I found "covert" as a verb instead of convert. And I've found many a truncated article.
These errors can raise costs in many ways. A couple I can think of are staff to read the complaints in email and the people who stop reading.
Well, I never make misteaks. (And sometimes spell checkers don't let me.) Somday I'll proof it to yuo.