In the episode where Jesse jumps out of the plane on his wedding day, you see him put on his helmet as he is telling Joey that if something happens to him, he wants joey to marry Becky, but he does not buckle it. The camera switches angles to outside immediately and the helmet is buckled. So yea i guess if he is super quick this could have happened..but if i was going to jump out of a plane I would take a little more time to make sure the helmet is on securely!