defect-escape-taxonomy

What this classifies

A defect is "an imperfection or deficiency in a work product where it does not meet its requirements or specifications or impairs its intended use" (ISTQB, defect). When a defect reaches production, the interesting question is not what the defect was. It is which checking activity was supposed to stop it and did not.

ISTQB has a formal term for exactly this: an escaped defect is "a defect that is not detected by a test activity that is supposed to find it" (ISTQB, escaped defect). That definition is the whole skill in one sentence. Classification means naming the test activity that was supposed to find it, then naming why that activity did not.