By recording the sound of a key being inserted into a lock, people can now 3D print a duplicate key.

Real world interactions produce physical artifacts in the form of heat, light, sound, etc. As our computing power and techniques continue advancing, our ability to detect and process ever more subtle signals leads to some unexpected results.

Found via @JeffDean's twittter feed. Excerpt:

It sounds unlikely, but security researchers say they have proven that the series of audible, metallic clicks made as a key penetrates a lock can now be deciphered by signal processing software to reveal the precise shape of the sequence of ridges on the key’s shaft. Knowing this (the actual cut of your key), a working copy of it can then be three-dimensionally (3D) printed.

Picking Locks with Audio Technology