re: Guess what did this damage to my son’s car.

quote:

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Slammed the door on the seatbelt buckle? Or some hard metallic thing.

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This is the answer. The reason the door doesn’t have any damage is because the force of the door was against the fat part of the buckle. The door jamb side was against the metal blade part of the buckle.

Have your son sit in the car with his seatbelt on and buckled. Get him to unbuckle it and prevent it from retracting. Have him get out of the car and see if the buckle hangs down far enough to rest in the door jamb. If it does, mystery solved.