Occasionally adult fontanel(le)s aren't of natural origin. Someone with a severe head injury (gunshot, equipment accident, etc.) as a small child may require reconstruction of the skull. Obviously as the skull grows, the artificial bits, if any, don't. So you can end up with a gap or lumpy area. These should generally be fully reconstructed at some point, but if the person in question isn't engaging in head-cracking activities, it's not generally urgent unless the gap places the brain in danger of damage from direct contact to that area of the head.