In a tricky night shot for his new flick “I Am Sam,” Sean Penn had to walk past a 7-Eleven over and over and OVER because the director couldn’t get the right lighting and angle. After hearing “Sorry, Sean . . . once more, please!” a whopping 37 times, the tired, irritated star FLIPPED! He suddenly turned his backside to the camera during a take, dropped his pants and MOONED! Cast and crew cracked up — and finally got a useable shot!