A double case study followed by some conclusions on dilligence of a physician in drawing inferences from already known facts. The cases come from so-called „medical courts” (i.e. disciplinary, not „real” courts provided by Constitution). The essence was: a physician involved performed a physical examination of a patient as well as an interview (anamnesis). The examination was complete and technically correct. The signs and symptoms found were sufficient to formulate a diagnosis, but a false negative error occured. In case (1.) the physician neither made an adequate diagnosis, nor recognized the situation as an emergency one. Case (2.) was similar, with an additional factor of differential diagnosis. In both cases „wrongness” did not lay in manual or perceptual elements of the examination, but in inference. Similar errors are named „logical”, but diagnostical matter is not purely logical; it is inevitably based on heuristic and an element of guessing. The idea of „due dilligence” is usually interpreted as conforming the activity performed with the adequate set of rules. Many medical actions and mental efforts are covered with rules but there are no rules of guessing. Probably all legal systems agree, that „non-guessing” can be a culpable „mental omission” and falls under responsibility. The idea is common, obvious and undobtful, but its rationale looks a little unclear. Beside this, some causal matters (or rather negative "quasi-causality"), probability and statistics, significant in the context of case (2.), are shortly discussed.