Not to be "that nerd" but it wasn't an accident. The writers actually performed a fairly elegant retconning to explain the difference in appearance between the Klingons as they appeared in multiple series.
It was an unintended side effect of attempting to use gene augmentation developed for humans, causing a mutation and disease to spread. They developed a cure with help from the crew of the enterprise/phlox but that cure didn't immediately fix the mutations, it took generations.
That's why I called it an accident.