I saw a documentary movie about this a couple of years ago. The Wikipedia article also has a fair amount of detail.

The company that built the tank was supposed to fill it with water to test it for leaks. The people responsible put about a foot of water in it & called it a day. When filled with molasses, it turned out to leak very badly, and people from the neighborhood routinely visited the leaks to collect free molasses. It is not known precisely what caused the final catastrophe, but with seams that bad it is no surprise.

The many people who sued the company not only won, but got their money! I suppose if such a thing happened today, a bankruptcy would have been wangled.