When I was an undergrad going to buy something at the campus chemical supply store, I noticed an open cabinet which had in it, among other things, a canister of chlorine gas... I remember thinking morbidly that I could just walk over and open the valve on that thing, and kill everyone in the room.
I still can't believe they didn't even have it locked up. I also remember that there was a back door to the chemical storeroom standing wide open at the time. I could have literally stuck it under my coat/in my backpack and walked right out and I'm sure no one would have even noticed. It was kind of appalling.