When it comes to choosing the ten commandments of copy editing, number one is a no-brainer:
Thou shalt not introduce errors into a text.
Just like a physician should do no harm to a patient, a copy editor should likewise do no harm to a text. (If you don’t appreciate the comparison of editing with the medical profession, I urge you to consider how authors feel about their
babies manuscripts — as they should!)
The editing of a text is done in a context of time and money. The two parties agree on a delivery date and a price, which are based on the complexity of the copy edit: thorough and precise or quick and dirty? If the latter, there will probably be a few errors that make it into the final product. No biggie. It’s not ideal, but you get what you pay for.
However, missing errors due to time and money constraints is not the same thing as introducing errors. Introducing an error goes against everything a copy editor stands for: clarity, correctness, and consistency. Errors reflect poorly on the writer and publisher and cause headaches for the reader. To willfully cause errors is thus anathema to editors. Editors are there to stop the bleeding, not to puncture another artery (had enough of the medical analogies yet?).
I guess a corollary is that an editor who introduces an error shall expose a lack of knowledge and shall be considered evermore a fool.
And nobody wants to be that guy.