I have seen several animals not drop, or, drop and then get up, following neck shots. I've always preferred neck shots in order to save meat, but shots have to be precise, especially on neck-swollen rutting mule deer. I spent an hour searching for a dry doe one evening after aiming where I thought spine was and watching her drop at the shot. After no luck in finding her, I realized I had only done what horseless frontiersmen had done on the southern plains: I had creased her and knocked her down but when she disappeared into the sagebrush I became distracted and did not notice when she got up and ran off.
