Because window flashing disappears from sight once the window itself is in place, it’s easy to rush through that part of the job and move on to something else. Windows are sometimes installed with no flashing at all, and in my work as a remodeler I often see windows that are installed right over the housewrap, with the adhesive membrane applied over both the housewrap and the window flange. That’s better than nothing, but not by much — it can’t stop any water that gets behind the housewrap above the level of the window, since by then water will already...