I tell students in my editing class at Loyola University Maryland that editing is like surgery.
Every time surgeons open up a human body, they create the possibility of doing damage to healthy tissue, of creating an injury in the attempt to cure one. Similarly, every time an editor opens up a text, the possibility of doing damage, of creating an error where none existed previously, is always there.
That is why in editing, as in surgery, the challenge is to do the absolute minimum necessary to correct the problem: the scalpel, not the ax. Sometimes massive reconstruction – or even amputation — may be called for, but always try not to go beyond what is absolutely minimally necessary. The more you do, the greater the chance of making a mistake.
Also: Keep in mind that the writer is not under anesthetic.
It's proverbial that doctors bury their mistakes. Editors publish theirs. And because editing occurs in the final stages before publication, an error by an editor is less likely to be caught than an error by a writer.