In a rare accident involving a special-effects scene, a special effects assistant received critical injuries Thursday as a handheld mock-up of a stinger missile exploded as it was being prepared to be used in a scene aboard a helicopter, the Associated Press reported Thursday. The explosion took place at Downey Studios, a former aircraft hangar in Downey, CA that had once been operated by Boeing and NASA, and which has now been converted into a film production facility. The assistant, whose name was not disclosed, was taken to a nearby hospital where he underwent surgery Thursday evening. The special-effects device was to have been used for a scene in Charlie Wilson's War, starring Tom Hanks and Julia Roberts, neither of whom were present at the time of the accident, the A.P. reported, citing Downey Fire Capt. Darren Moon.