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It is, but it shouldn't be the only edit that happens to a manuscript. I send mine off to an editor once I've finished it.

Should still be the best you can manage, but it often takes another pair of eyes to see things you've missed. Doesn't always work, though. I once had a publisher's proof reader put a mistake INTO my book. I'd correctly written computer 'program' and she changed it to 'programme' throughout. So everyone who read it would have thought it my own stupid error - duh!

When I edit or proofread, I make certain the author knows that the changes are all advisements - It's up to the author to go over the work I've done and either accept or reject every alteration I suggest.

If that had been the case with your proofreader, you would have picked up the error and it wouldn't have gone through.

Should still be the best you can manage, but it often takes another pair of eyes to see things you've missed. Doesn't always work, though. I once had a publisher's proof reader put a mistake INTO my book. I'd correctly written computer 'program' and she changed it to 'programme' throughout. So everyone who read it would have thought it my own stupid error - duh!