OK, sad story. The last few weeks there have been spelling errors in a couple of things that went out of the marketing department. MY marketing department. Nothing too serious, but then again, is there ever a good mistake. No matter how minor, to some, a spelling mistake is as big as has ever existed. What I do for proofreading is find the most detail oriented paralegal at my firm and make sure everything passes under her/his eyes. A few weeks ago "that person" at my firm was out so I fielded the proofreading to someone different. It didn't turn out so well. One vowel in the wrong place was the result. Ouch! Taking a professional ding is never pleasant.
Here is my new rule: New proofreaders are validated with additional proofreaders. I proof it, then I send it to the new proofreader, then I send it to someone else for proofing, and then I send it to one more.
It is so weird the spelling and punctuation errors that slip by... they're like little time bombs that only go off when they can do the most damage. And at law firms where every "t" and "i" need to be perfect -- OOF! Need I say more. Pay attention to the details, and pay attention to who is paying attention to getting the details right. Your credibility stands in the balance.