An unusual mechanism of gross backstreaming has been recently observed in diffusion-pump systems with liquid-nitrogen traps. It occurs during or following the charging of the trap with liquid nitrogen and can be recognized by the appearance of small droplets upstream from the trap and by accompanying pressure fluctuations. The amount of pumping fluid which may reach the baseplate of a typical bell-jar vacuum system due to this mechanism may be few orders of magnitude greater than the normally expected steady-state backstreaming rate. The phenomenon is produced by fracture of the frozen pumping fluid film due to the unequal thermal expansion coefficients between it and the metal surfaces on which it has been previously deposited. The elastic energy stored in the film is sufficient to impart high velocity to the fragments resulting from the fracture. The paper describes the observations and suggests methods of prevention.