This is an interesting thought to me (like, how does one create a zero-day that doesn't look intentional?) but the more I think about it, the more I start to believe that this fully is not necessary. There are enough faulty humans and memory unsafe languages in the loop that there will always be a zero-day somewhere, you just need to find it.
(this isn't to say something like the NSA has never created or ordered the creation of a backdoor - I just don't think it would be in the form of an "unintentional" zero-day exploit)