COLUMBIA UPDATE: Reportedly, it was a flaw in the foam-gluing process that led to the loss of the space shuttle Columbia:

The fault apparently was not with the chemical makeup of the foam, which insulates the tanks and prevents ice from forming on the outside when 500,000 gallons of supercold liquid hydrogen and liquid oxygen are pumped aboard hours before liftoff.

Instead, Otte said NASA concluded after extensive testing that the process of applying some sections of foam by hand with spray guns was at fault.

Gaps, or voids, were often left, and tests done since the Columbia accident have shown liquid hydrogen could seep into those voids. After launch, the gas inside the voids starts to heat up and expand, causing large pieces of insulation to pop off.

Sometimes it’s the little things that get you. (Via Slashdot, which has some interesting comments.) And here’s an article on related issues regarding Shuttle safety.