This pisses me off every time. Being shot in the back from close range drives me up the wall, especially when its an experienced player who is doing it. It especially gets me when they, and in my experience it has always been noobs, wait until you are 3 or 4 feet away before they shoot you where its gonna hurt.
"Do not touch anything unnecessarily. Beware of pretty girls in dance halls and parks who may be spies, as well as bicycles, revolvers, uniforms, arms, dead horses, and men lying on roads -- they are not there accidentally."
Soviet infantry manual, issued in the 1930's