A team of researchers from the U.S. and Europe has developed a Hardware Trojan, which is an undetectable to many techniques, raising the question on need of proper hardware qualification. The Scientists devised two such backdoors they said adversaries could feasibly build into processors to surreptitiously bypass cryptographic protections provided by the computer running the chips. Their modifications fooled a number of common Trojan testing methods that included optical inspection and checking against golden chips. They inserted their hardware Trojans by changing the dopant polarity of existing transistors.
Source: https://thehackernews.com/2013/09/Undetectable-hardware-Trojans.html