Cognitive processing associated with the recognition of computer malfunction was examined by recording event-related brain potentials time-locked to voluntary mouse button clicks that did not produce feedback from the computer. Sixteen university students performed a simple stimulus discrimination (oddball) task in auditory and visual modalities. Each button press produced either a target or a standard stimulus, or no stimulus response in some trials (missing stimulus, p=0.2). Biphasic missing-stimulus potentials appeared after the stimulus omission within a latency range of 150 to 550 ms. For both auditory and visual modalities, the initial negative component was dominant at the right temporal site, whereas the following positive component was at the central site. The peak latencies of these potentials were about 30 ms shorter in the auditory than in the visual modality, though the eliciting event was identical (i.e., stimulus omission). Brain potentials after users’ operations may provide an objective and unobtrusive index of users’ expectations in human-computer interaction.