Here's a mistake I made a few years ago. The question I wanted to answer was "How do you decide how much UV you should get?". Now it turns out that your skin colour is correlated with the Autumn UV levels where your ancestors come from. We are optimized for sunlight levels where our genes come from. So I thought that that level of sunlight is what will be best for us.
It was a particularly dumb case to get wrong. The English may be designed to survive a UV-free dull grey winter. But that doesn't mean that it is optimal for them to experience that. It actually seems extremely likely that, however pigmentless your skin, it is a good idea to get some extra UV in some way during winter, or at least take vitamin D supplements. Extremely likely but not certain. Maybe our genes have developed some trick that depends on that break in UV exposure to work correctly.
This error is related to the logic error called Modus Tollens, and humans seem particularly prone to errors of this general type. And we can see why. If there is absolutely no other factor that you can think of then it is a reasonable starting point to assume that what we are optimized for is (arguing backwards) what is optimal for us. Still, as we see, we can nearly always think of other factors.
A case where this style of argument is being used is the Paleo Diet. Maybe we are optimized for a paleo diet. That doesn't mean we can't improve on it. It does mean that we shouldn't reject aspects of it too quickly.