the.com/human element

means The unpredictable factor of human judgment, emotion, or error that no system, process, or plan can fully eliminate.

from A straightforward compound of 'human' (from Latin humanus, 'of man,' itself tied to humus, 'earth' — mortal, earthbound stuff) and 'element,' from Latin elementum, one of the basic components the ancients thought everything was built from. Put together, the phrase is a 20th-century habit of speech — business, engineering, and military writing reaching for a tidy label for the one variable in any system that refuses to behave like a variable.