Your bathroom exhaust fan blows directly into your attic.

As crazy as it sounds, historically most homebuilders have installed that power bathroom exhaust fan in your second story bathroom failing to direct the warm moist air from your bathroom anywhere but in your attic.  The science of creating the right atmosphere in your attic space did not really become a focus of homebuilders until the last 8-10 years.  If your attic has been done properly, the exhaust from your bathroom is directed through an insulated vent-line directly out through a dedicated vent on your roof.  Over the years some contractors have used methods to cut corners.  You will find many homes with tin lines that direct the exhaust up towards a mushroom roof vent or out to a soffit, however this still allows a certain percentage of the moist air to make its way back into the attic causing damage and often times, attic mold.