The assumption that intelligent people cannot simultaneously be athletic or fun-loving or want to do other things besides study.
That comes only second to the assumption that because I do something without hesitation, fear, or concern I must not be thinking about it.
I think about things all the time. Training in parkour and other physical activities as well as simple observation lets me know my own skills and limits very well. So when I climb a loose pile of rocks or sit on a high outcrop or run through a thick trail I have already judged where I have to step to avoid shifting the rocks, or made sure I have a firm handhold and a short and/or soft fall if I slip, or I have seen the safest path on the trail; but because I do this quickly everyone tells me I need to be more careful or think about what I'm doing, ignoring that I already have many times over.