People are better than we give ourselves credit for. I remember working at Disneyland during 9/11, the FBI told us we were considered the 4th most likely target in the United States, and mandatory bag checks at the gates made the lines longer and slower than they'd ever been, so enormous crowds, thousands of people, were gathered and effectively trapped between Harbor Blvd and the actual gates. I couldn't help but think how easy it would be for a few guys or even just one to gun down hundreds with a couple of uzis or throw a few pipe bombs into the crowd. Nobody ever did. That Las Vegas concert shooting was the closest anyone has ever come. There are probably many vulnerabilities like that. Poison the water supply to a hospital. Close off a mine on purpose. My girlfriend's brother back in 2004 managed to drive his speedboat with me in it through the LA harbor, right alongside container ships, and succeeded in evading the Coast Guard. It was just a joyride to him, but someone more malicious could probably sink a container ship or even an oil tanker.

These things almost never happen because people just aren't that evil, at least most of the time. When it happens, it's an extraordinary event.