If you had done large software projects you would know the answer. Reason is that coders copy software because of its functionality. What makes you think they know about the bugs? Most bugs are not obvious. As in a previously cited gene for susceptibility to scurvy in chimps and apes. As long as the chimps and apes live in an environment with plenty of fruit or vegetation that contains vitamin C -- the defect is not obvious at all.
That gene used to be functional in lower mammals. The designer must have known it was there and worked at some point since he/she must have made it. Did it just naturally deteriorate in the new environment since there was no selective pressure? Or did the designer just throw in a broken gene for the hell of it?