"Given the scale of the global credit card networks, it is almost impossible to detect every kind of attack, " he said.

WSJ: Global network of hackers steal $45m from ATMs

"Even so, notwithstanding the social audit, it would have been impossible to detect the structural defects of the collapsed building, " the company said.

WSJ: Doomed Factories Raced to Fill Orders

When taste buds become dry, they also become less receptive and your sense of smell is blunted, making subtle flavours nearly impossible to detect.

BBC: A possible end to plane food

Many viruses are harmless in their regular hosts (and therefore difficult or impossible to detect) but devastating if they switch to a new one.

ECONOMIST: Pigged out

At first, it was almost impossible to detect but easily correctable.

FORBES: USC's Fall From College Sports Elite A Cautionary Tale

Even though we were in a hybrid, the gasoline engine kicked on several times and it was nearly impossible to detect an increase in noise from within the cabin.

FORBES: Test-driving The New 2012 Toyota Camry Hybrid

The size of this harm, which is larger than that documented for Vioxx, is small enough that it would be nearly impossible to detect in the studies that were required for approval.

FORBES: The FDA Is Faster. Now Let's Make It Safer

New technologies--including impossible-to-detect gene therapies--are almost certainly on the horizon.

FORBES: The Drug Test Bait And Switch