Some of the methods of genome editing are now so efficient and precise that I do not think it is a great concern, but you always have to check. There are good ways of checking what you have done and what you have got. I would not be that concerned. You would have to check the original animal that has been modified, but once you get to subsequent generations, you will be pretty certain of exactly what you have, and of anything wrong. The methods are being used in humans for somatic genome editing. We know a lot about them and how accurate and safe they can be.