Or rather, a supposed instrument for looking at things in hindsight. An arthroscope looks into joints, a laparoscope into the torso, the retrospectoscope into the restrospect. Started as medical slang, but I've seen it elsewhere without any sense of its origin. Note that the coinage mixes Latin and Greek roots, for added jocularity.
Everyone's vision is 20-20 through the retrospectoscope.