Endermite spawned when an endermite was enraged and teleported towards the player. Skeleton shot at the player but hit a block above the endermite. block was removed and the skeleton's Arrow fell on the endermite. The endermite began to spin, couldnt be killed, couldn't be tracked by other endermen or hurt the player.
This issue has been reported already. It has been linked to the original bugreport.
Please search before reporting any bugs, as it's likely that one exists already.