Humans typically overestimate the perceived duration of the initial and final event in a stream of identical events. This oddball effect may serve an evolutionarily adapted "alerting" function and is consistent with reports of time slowing down in threatening situations. The effect seems to be strongest for images that are expanding in size on the retina, i.e., that are "looming" or approaching the viewer, and the effect can be eradicated for oddballs that are contracting or perceived to be receding from the viewer. The effect is also reduced or reversed with a static oddball presented among a stream of expanding stimuli.