In Ars' series on the Apollo program Eric tells a story about the Lunar Module from NASA lore...
...recounted in Apollo: The Race to the Moon, a technician working with a test version of the vehicle absentmindedly clicked the cap of his ballpoint pen against a filled fuel tank. This minute amount of pressure opened a small leak in the tank, and the thin stream of propellant sliced off his finger with surgical precision.
Did this really happen? And if so how was the tank designed to be strong enough to withstand take-off, but so weak another way?
Tom Kelly's book talks a lot about the weight saving efforts and the strict QA around the tanks due to the danger they could pose (going so far as to reject a tank that passed all the tests because it was inexplicably off-colour!) but it seemed this was precisely so they would be anything but fragile..