Which historical warship design error should be prevented?

If you could prevent one historical warship design error, what would it be?

A high-impact stability or armament-development mistake would be the best target because it affects survival and whole generations of ships.

The most consequential errors damaged survivability or pushed a useful design into the wrong role. Candidates include the Littoral Combat Ship's extreme-speed requirement and side arrangements that made some small British cruisers capsize quickly after damage. A faster Nelson-family battleship with 15-inch guns is preferred to the historical compromise, while Alaska might have survived as a programme if 9.2- or 10-inch guns had kept her clearly in the cruiser category. Earlier commitment to high-angle dual-purpose destroyer guns could also have delivered the 4.5-inch system sooner. Nominal armour type and thickness cannot be separated from manufacturing quality and later metallurgical development.