The best example I've seen this IB is a guy who seemed impervious to damage from our team landing on our Rift and the spark not registering with it,he kept running in and out multiple times trying to ignite our Rift to no avail while we where doing our best to kill him. In the end he must of realized that it wasn't going to work so all of a sudden he became mortal and we battered him to death with our melees.