 Performable errors clearly emerge only at the execution level, and they correspond with Turing’s (1950) error of functioning, also called operational malfunctions. Conceptual and material errors may arise at any level of abstraction from the intention level down to the physical implementation level. Conceptual errors engender mistakes, while material errors induce failures. For instance, a mistake at the intention level consists of an inconsistent set of requirements, while at the physical implementation level it may correspond to an invalid hardware design (such as in the choice of the logic gates for the truth-functional connectives). Failures occurring at the specification level may be due to a design that is deemed to be incomplete with respect to the set of desired functional requirements, while a failure at the algorithm level occurs in those frequent cases in which the algorithm is found not to fulfill the specifications. Beyond mistakes, failures, and operational malfunctions, slips are a source of miscomputations at the high-level programming language instructions level: they may be conceptual or material errors due to, respectively, a syntactic or a semantic flaw in the program. Conceptual slips appear in all those cases in which the syntactical rules of high-level languages are violated; material slips involve the violation of semantic rules of programming languages, such as when a variable is used but not initialized.