On the approach to generation 300, the run was suddenly stricken with an epidemic of sliders (creatures who lack pose genes and thus can't move; if you try leading them with the hand they "slide" along the ground, hence the name) and other such maladies. And among the individuals in the run was GrM0262 (a clinical name given to him by Kezune's Wolfing Autonamer, which I used to track generations in the time before I made my Wolfling Nametagger).
All of the genomes I used to start the run and continued to add new generation 1 creatures from were CFE based and included all of their immortality preventing edits. Yet somehow, GrM0262 was immortal; he was going on 17 hours of age for genomes that died under the five hour mark. I naturally exported him as soon as I realized and set about trying to figure out what went wrong.
At the time, I never did, and the best anyone else could figure out was that he was extremely vulnerable to injury and basically all of his organs were dead. Ten years later, I came to a realization about what the cause could've been...and not only was it figured out, but it lead to the discovery of yet another oversight with the vanilla genomes.
