yelnif,
From what I'm told by the experts, sometime the metal of the coin stretches and the hit portion remains attached but relocated, but sometimes the portion that got hit if there is any detachment can get sorta glued pretty well in place by elements of the coin's environment over many years in circulation or being in the ground, e.g. a combination of dirt, body oil, and any other thing that can act as an adhesive, and in that case the affected portion maybe has only one solid attachment point after the hit, but probably not as delicate as "hanging on by a thread" since it is metal and still rather strong.