This paper is concerned with the claim that epistemic terms and categories are historical entities. The starting point is the observation that recent attempts at historical studies of epistemic terms fail to bridge the gap between history and philosophy proper. I examine whether, and how, it is possible to forge a closer link between historical and philosophical aspects of conceptual analysis. The paper explores possible links by analyzing aspects of the concept of error. A "pragmatic" and a "mentalist" notion of error are identified in current philosophical studies of error: according to the latter, errors can be ascribed only to mental operations, according to the former, errors can also be ascribed to things and processes. The paper then draws on historical accounts of optical instruments to highlight certain presuppositions and implications of these two uses of the term. Contextual features and trans-contextual structures of the notion of error are distinguished. In conclusion, I argue that an intimate link between history and philosophy of science can be forged by an analysis ofthe development of conceptual arrangements which allows for trans- contextual structural aspects while drawing attention to the contextual epistemological and scientific conditions of their re-arrangement.