Paleoclimate data increasingly suggest the likelihood of abrupt transitions due to instabilities in the climate system. Several previous studies offer support for the snow/ice instability mechanism as an explanation for some of these changes. However, most of these studies have either employed simple models or not closely examined the details of the transition. Herein we revisit this issue using a general circulation model (GCM) simulation for the late Precambrian, a time when glaciers may have reached the equator. Our results indicate that for CO2 concentrations near present levels the snow/ice instability occurs in one model year. The width of open water associated with the onset of instability agrees with theoretical calculations of the critical length scale predicted for this behavior. These results strengthen support for the existence of this phenomenon as a mechanism for rapid climate change.