ERPs elicited by stimulus-driven retrieval of arithmetic facts related to multiplication were investigated. We recorded the electrophysiological activity from the scalp of participants while they were performing a number-matching task. Crucially, arithmetic was task-irrelevant within this paradigm, because participants were simply to physically compare a cue composed of two one- or two-digit numbers and a single target number. In accordance with previous studies, behavioral data showed that, in non-matching trials, participants were significantly slower and less accurate to respond when the target number was the product of the two numbers in the cue compared to when the target number was arithmetically unrelated (i.e., neutral) to those numbers (interference effect). In line with recent findings on ERPs associated with task-relevant arithmetic facts retrieval, we showed that the interference effect resulted in a modulation of the amplitude of an N400-like ERP component, with neutral targets generating relatively more negativity than product targets. The observed dissociation between behavioral data and ERP measures is interpreted as evidence of activation spreading in the lexicon of arithmetic facts, since alternative explanations relying on strategic factors such as expectancy or semantic matching would have predicted the two measures to show a converging trend.