The reality is that in any organism, there are birth defects. We are very prone to thinking it is a morph issue or incubation issue, as there has been so much documented in both cases. Nevertheless, when everything is perfect and the morphs aren't known to be predisposed to complications, will still see defects and deformities. It's the nature of nature. There is the standard, and then there are the anomalies.
This year I hatched 4 clutches that totaled 20 eggs and two of the twenty were boob eggs. I incubate at lower temps, about 87F. Everything came out beautiful, including the boob eggs, which were just very small because there wasn't as much mass to start with. One was 30 grams and one was 40 grams. However, I had one clown that came out looking fine other than it's head seemed a little small for the body. Not a pencil head, but just disproportionate by maybe 10% or 15%. It is having difficulties with locomotion. It can only move forward for the most part and cork screws badly. It does not withdraw from being touched on the nose, and it is almost never coiled but usually lies somewhat extended, if it isn't wedged with its head in the upper corner of the tub. I will be euthanizing it. There is no real reason for it. It just happened.