If you're very young or very old you could be misjudged for not being as capable as somebody in their 20s for example. While this may often be true it is not always and isn't something we should strive towards.
If you're of a certain ethnicity; let's say you're a Kenyan and you're running a marathon, people could misjudge that you would run the marathon at a very good pace. This, again, is not the necessary case.
If you're female, it is often possible that you would be misjudged as not being strong enough compared to your male counterparts.