Just played a match where the enemy Karthus was untargetable (could not right or left click the champion) many times through out the game.
It also happened to the enemy turret where I could not target the turret.
Other players on my ally team also not target the Karthus, but they could target the tower.
This happened to me against a kogmaw, after minute 35 the all team couldnt target kogmaw. We couldnt win any team fights because he was always alive. Some of us had to relog to be able to target him.
© 2014 Riot Games, Inc. All rights reserved. Riot Games, League of Legends and PvP.net are trademarks, services marks, or registered trademarks of Riot Games, Inc.