From: Groningen, NL
Heralded as the greatest warship of its time, the Vasa sank within 20 minutes of its maiden voyage. Nearly 400 years later, no-one knows why.
People know why this happened, and a month before the event a standard stability test - men running from one side of the ship to the other, then the reaction was recorded - failed. It even had to be broken off to prevent capsizing.
Too little ballast, too heavy calibre guns on the second deck: it killed the inherent stability, which was tricky for the warships at the time anyway. No fault of the ship itself, but faulty arrangement of equipment. But mostly epic failure in the chain of command.
Incredibly, given the failed test, it sailed its maiden voyage with open gun ports.