The detection of fast astronomical transients in photographic plates from the Palomar sky surveys conducted in the 1950s, was subject to the criticism that such transients could be just the effect of otherwise unaccounted for plate artifacts. In this paper, we show that transient images exhibit the coma aberration pattern expected from off-axis point sources recorded through the telescope optics, a signature that plate artifacts cannot naturally reproduce. Although the data does not by themselves establish the physical origin of the light that generated the images, they lend support to hypotheses that do not rely on instrumental effects to explain transients.