VIDEO : Amy Smart Movie Shut Down Due to Live Grenade!

Amy Smart?'s indie movie's already blowing up -- well, almost -- since filming is on hold right now after a real GRENADE was found on set! The bomb scare began when location scouts showed up early Friday morning in an Altadena, CA neighborhood to begin work on "Sister Cities" ... which stars Smart and Alfred Molina. Director Sean Hanish tells TMZ that the scouts stumbled upon what looks like an old, rusted grenade ... sitting in a driveway. They immediately called on-set fire officials. Once police got there, they decided better safe than sorry -- and phoned in the bomb squad. The bomb squad later showed up and identified the object as a WWII era training grenade, while also adding that it wasn't explosive. Sources say the set has been cleared to resume filming.