Vertical tail surfaces
In reports of flight testing and development over the years from the First World War to the present day, there is a continuous thread of aircraft requiring enlargement of the vertical tail surfaces following initial prototype testing. Aircraft continue to sprout dorsal/ventral fins etc to this day and yet I haven't heard or read of any that have needed a reduction in vertical surface area. Why the reluctance to design them large enough in the first place? OK, I can see that structural weight is a consideration, but it must be more difficult to redesign the structure to accommodate subsequent enlargements.