When a short‐pulse laser beam is focused onto a solid target in a few torr ambient gas, the expanding target plasma couples to the resulting ambient plasma. Dark‐field shadowgrams show that a thin, nearly spherical blast front is produced. These shadowgrams also show that certain regions of the blast front (called aneurisms) may project well ahead of the neighboring, spherical part. Several mechanisms for aneurisms are discussed. One class of aneurisms is consistently produced along the laser axis when the laser energy is greater than about 10 J. Two‐dimensional, hydrodynamic computer simulations of these on‐axis aneurisms show that they can be accounted for by the laser heating of an on‐axis channel.