A bit of a both and more. The most often case is I can't transform at all, but if I do manage, the form might not hold for very long or it could be the wrong size. Some other...abnormalities would be slightly deformed limbs, such as shorter legs or missing a tail. If the only issue is size I can hold the transformation with more effort, but if there are other inaccuracies then it won't last for more than a few seconds.