Traditional approaches to the modeling of multiple-choice item response data (e.g., 3PL, 4PL models) emphasize slips and guesses as random events. In this paper, an item response model is presented that characterizes both disjunctively interacting guessing and conjunctively interacting slipping processes as proficiency-related phenomena. We show how evidence for this perspective is seen in the systematic form of invariance violations for item slip and guess parameters under four-parameter IRT models when compared across populations of different mean proficiency levels. Specifically, higher proficiency populations tend to show higher guess and lower slip probabilities than lower proficiency populations. The results undermine the use of traditional models for IRT applications that require invariance and would suggest greater attention to alternatives.