Correlational designs can get messy. We observe the relationship between two variables, often without taking in to account the non-controlled nature of such a design, and we leave out the fact that many other variables may be at play, affecting the relationship between those two variables. Sometimes researchers take this in to account, and sometimes they don't. Please provide a real or made-up example of when a correlational design may be problematic or even detrimental if researchers do not consider other variables at play.