I have inter bred its usually ok for about 3 or 4 generations, anything after that you risk deformity such as bent tail spine, I've had a few if these - I cull them as soon as I see them as it effects the way they swim and effects the overall look of the tank bent fish struggling to swim in a straight line,
This caption and pic should help :-
The only real deformity that I can think of is a spinal one and I think it is common practice to eliminate fry with a spinal deformity. With the over breeding, traits would change much in the way they would if you were line breeding for specific traits such as fin length or color.