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Durring a summoners rift game with an opposing Karthas he became untargetable half way through the match. He was solo center with teleport and revive. Once we had Soraka kill him and we were able to target him again. Later in the match we killed him once more and he became untargetable until a tower took him down. I don't know what caused this as our team thought it was a revive glitch until towards the end of the game we managed to kill him and his revive left him still able to be targeted.