I remember a colleague once edited someone’s copy – and inserted FIVE serious errors into it by accident. He changed a Mrs to a Miss and a country from Venezuela to Vietnam. A good day at the office for him!
He argued that he’d removed far more errors than he added. But that wasn’t the point. Copy should be perfect once an editor has finished with it.
But it is easy to insert new mistakes when you’re editing – I’ve done it many times. Errors can occur, especially if you’re interrupted halfway through doing something, or when you’re copy and pasting bits of copy from one place to another.
I find it’s best to proofread the final, edited version, to make sure nothing’s been changed or left out. Compare it with the original.
It also helps to highlight material in the original copy that you changed or left out. This makes it easier to check later on.